refactor: replace null-forgiving operator with explicit null check

Remove null-forgiving operator (!) in DataReaderExtensions and add defensive null check with descriptive exception. While deserialization failure is unlikely (we control the JSON construction), an explicit check provides better error messages if data corruption or type mapping issues occur.

Author: Per Kops

src/Atc.Kusto/Extensions/DataReaderExtensions.cs

@@ -67,7 +67,15 @@ public static T[] ReadObjects<T>(
             doc.WriteEndObject();
             doc.Flush();
 
-            results.Add(JsonSerializer.Deserialize<T>(buffer.WrittenSpan, options)!);
+            var deserializedObject = JsonSerializer.Deserialize<T>(buffer.WrittenSpan, options);
+            if (deserializedObject is null)
+            {
+                throw new InvalidOperationException(
+                    $"Failed to deserialize DataReader row to type {typeof(T).Name}. " +
+                    $"This may indicate corrupted data or an incompatible type mapping.");
+            }
+
+            results.Add(deserializedObject);
         }
 
         return [.. results];