Use default document type if no main part is available (dotnet#1836)

Fixes dotnet#1745

Author: twsouthwick

src/DocumentFormat.OpenXml.Framework/Resources/ExceptionMessages.Designer.cs

@@ -240,9 +240,6 @@
   <data name="MultipleRelationshipsToSamePart" xml:space="preserve">
     <value>There are more than one relationship references that target the specified part.</value>
   </data>
-  <data name="NoMainPart" xml:space="preserve">
-    <value>The specified package is invalid. The main part is missing.</value>
-  </data>
   <data name="NonCompositeNoChild" xml:space="preserve">
     <value>Non-composite elements do not have child elements.</value>
   </data>

src/DocumentFormat.OpenXml.Framework/Resources/ExceptionMessages.resx

@@ -39,12 +39,10 @@ private TDocumentType EnsureDocumentType()
             return existing;
         }
 
-        var hasRelationships = false;
         var package = this.GetRequired<IPackageFeature>().Package;
 
         foreach (var relationship in package.Relationships)
         {
-            hasRelationships = true;
             if (relationship.RelationshipType == RelationshipType)
             {
                 var uriTarget = PackUriHelper.ResolvePartUri(OpenXmlPackage.Uri, relationship.TargetUri);
@@ -55,14 +53,10 @@ private TDocumentType EnsureDocumentType()
             }
         }
 
-        if (!hasRelationships)
+        // If document type hasn't been set by content, start with the TDocumentType default value
+        if (!_documentType.HasValue)
         {
-            _documentType = default;
-        }
-
-        if (_documentType is null)
-        {
-            throw new OpenXmlPackageException(ExceptionMessages.NoMainPart);
+            _documentType = default(TDocumentType);
         }
 
         return _documentType.Value;

src/DocumentFormat.OpenXml/Packaging/TypedPackageFeatureCollection.cs

@@ -570,6 +570,26 @@ public void PresentationDocumentTypeIsRetrievedIfAvailable()
             Assert.Equal(PresentationDocumentType.Template, doc.DocumentType);
         }
 
+        [Fact]
+        public void DocumentTypeUsesDefault()
+        {
+            // Arrange
+            using var ms = new MemoryStream();
+
+            // Note - if a main part is not created, then no type is persisted
+            using (var emptySpreadsheet = SpreadsheetDocument.Create(ms, SpreadsheetDocumentType.MacroEnabledTemplate))
+            {
+            }
+
+            ms.Position = 0;
+
+            // Assert/Act
+            using (var result = SpreadsheetDocument.Open(ms, true))
+            {
+                Assert.Equal(default, result.DocumentType);
+            }
+        }
+
         /// <summary>
         /// ChangeDocumentTypeInternalTest.
         /// </summary>