Root element for generated parts should be nullable (#1922)

Changes root elements for generated parts to be nullable. 
Fixes #959 and #995

Author: tomjebo

gen/DocumentFormat.OpenXml.Generator.Models/Generators/Parts/PartWriter.cs

@@ -22,6 +22,7 @@ public static class PartWriter
         "DocumentFormat.OpenXml.Framework",
         "System",
         "System.Collections.Generic",
+        "System.Diagnostics.CodeAnalysis",
     };
 
     private static readonly BlockOptions _options = new()
@@ -410,7 +411,7 @@ private static IEnumerable<Item> GetSchemaTypedParts(OpenXmlGeneratorServices se
             writer.WriteDocumentationComment("Gets or sets the root element of this part.");
             writer.Write("public ");
             writer.Write(apiName);
-            writer.Write(" ");
+            writer.Write("? ");
             writer.WriteLine(api.Class);
 
             using (writer.AddBlock(_options))

generated/DocumentFormat.OpenXml/DocumentFormat.OpenXml.Generator/DocumentFormat.OpenXml.Generator.OpenXmlGenerator/Part_AlternativeFormatImportPart.g.cs

@@ -9,6 +9,7 @@
 using DocumentFormat.OpenXml.Framework;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace DocumentFormat.OpenXml.Packaging
 {

generated/DocumentFormat.OpenXml/DocumentFormat.OpenXml.Generator/DocumentFormat.OpenXml.Generator.OpenXmlGenerator/Part_CalculationChainPart.g.cs

@@ -9,6 +9,7 @@
 using DocumentFormat.OpenXml.Framework;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace DocumentFormat.OpenXml.Packaging
 {
@@ -32,7 +33,7 @@ internal CalculationChainPart()
         /// <summary>
         /// Gets or sets the root element of this part.
         /// </summary>
-        public DocumentFormat.OpenXml.Spreadsheet.CalculationChain CalculationChain
+        public DocumentFormat.OpenXml.Spreadsheet.CalculationChain? CalculationChain
         {
             get
             {

generated/DocumentFormat.OpenXml/DocumentFormat.OpenXml.Generator/DocumentFormat.OpenXml.Generator.OpenXmlGenerator/Part_CellMetadataPart.g.cs

@@ -9,6 +9,7 @@
 using DocumentFormat.OpenXml.Framework;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace DocumentFormat.OpenXml.Packaging
 {
@@ -48,7 +49,7 @@ private protected override OpenXmlPartRootElement? InternalRootElement
         /// <summary>
         /// Gets or sets the root element of this part.
         /// </summary>
-        public DocumentFormat.OpenXml.Spreadsheet.Metadata Metadata
+        public DocumentFormat.OpenXml.Spreadsheet.Metadata? Metadata
         {
             get
             {

generated/DocumentFormat.OpenXml/DocumentFormat.OpenXml.Generator/DocumentFormat.OpenXml.Generator.OpenXmlGenerator/Part_ChartColorStylePart.g.cs

@@ -9,6 +9,7 @@
 using DocumentFormat.OpenXml.Framework;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace DocumentFormat.OpenXml.Packaging
 {
@@ -32,7 +33,7 @@ internal ChartColorStylePart()
         /// <summary>
         /// Gets or sets the root element of this part.
         /// </summary>
-        public DocumentFormat.OpenXml.Office2013.Drawing.ChartStyle.ColorStyle ColorStyle
+        public DocumentFormat.OpenXml.Office2013.Drawing.ChartStyle.ColorStyle? ColorStyle
         {
             get
             {

generated/DocumentFormat.OpenXml/DocumentFormat.OpenXml.Generator/DocumentFormat.OpenXml.Generator.OpenXmlGenerator/Part_ChartDrawingPart.g.cs

@@ -9,6 +9,7 @@
 using DocumentFormat.OpenXml.Framework;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace DocumentFormat.OpenXml.Packaging
 {
@@ -69,7 +70,7 @@ private protected override OpenXmlPartRootElement? InternalRootElement
         /// <summary>
         /// Gets or sets the root element of this part.
         /// </summary>
-        public DocumentFormat.OpenXml.Drawing.Charts.UserShapes UserShapes
+        public DocumentFormat.OpenXml.Drawing.Charts.UserShapes? UserShapes
         {
             get
             {

generated/DocumentFormat.OpenXml/DocumentFormat.OpenXml.Generator/DocumentFormat.OpenXml.Generator.OpenXmlGenerator/Part_ChartPart.g.cs

@@ -9,6 +9,7 @@
 using DocumentFormat.OpenXml.Framework;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace DocumentFormat.OpenXml.Packaging
 {
@@ -44,7 +45,7 @@ internal ChartPart()
         /// <summary>
         /// Gets or sets the root element of this part.
         /// </summary>
-        public DocumentFormat.OpenXml.Drawing.Charts.ChartSpace ChartSpace
+        public DocumentFormat.OpenXml.Drawing.Charts.ChartSpace? ChartSpace
         {
             get
             {

generated/DocumentFormat.OpenXml/DocumentFormat.OpenXml.Generator/DocumentFormat.OpenXml.Generator.OpenXmlGenerator/Part_ChartStylePart.g.cs

@@ -9,6 +9,7 @@
 using DocumentFormat.OpenXml.Framework;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace DocumentFormat.OpenXml.Packaging
 {
@@ -32,7 +33,7 @@ internal ChartStylePart()
         /// <summary>
         /// Gets or sets the root element of this part.
         /// </summary>
-        public DocumentFormat.OpenXml.Office2013.Drawing.ChartStyle.ChartStyle ChartStyle
+        public DocumentFormat.OpenXml.Office2013.Drawing.ChartStyle.ChartStyle? ChartStyle
         {
             get
             {

generated/DocumentFormat.OpenXml/DocumentFormat.OpenXml.Generator/DocumentFormat.OpenXml.Generator.OpenXmlGenerator/Part_ChartsheetPart.g.cs

@@ -9,6 +9,7 @@
 using DocumentFormat.OpenXml.Framework;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace DocumentFormat.OpenXml.Packaging
 {
@@ -33,7 +34,7 @@ internal ChartsheetPart()
         /// <summary>
         /// Gets or sets the root element of this part.
         /// </summary>
-        public DocumentFormat.OpenXml.Spreadsheet.Chartsheet Chartsheet
+        public DocumentFormat.OpenXml.Spreadsheet.Chartsheet? Chartsheet
         {
             get
             {

generated/DocumentFormat.OpenXml/DocumentFormat.OpenXml.Generator/DocumentFormat.OpenXml.Generator.OpenXmlGenerator/Part_CommentAuthorsPart.g.cs

@@ -9,6 +9,7 @@
 using DocumentFormat.OpenXml.Framework;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace DocumentFormat.OpenXml.Packaging
 {
@@ -32,7 +33,7 @@ internal CommentAuthorsPart()
         /// <summary>
         /// Gets or sets the root element of this part.
         /// </summary>
-        public DocumentFormat.OpenXml.Presentation.CommentAuthorList CommentAuthorList
+        public DocumentFormat.OpenXml.Presentation.CommentAuthorList? CommentAuthorList
         {
             get
             {