CSHARP-5119: FullDocument of the ChangeStreamDocument<TDocument> should not throw on BsonNull (#1341)

Author: sanych-sun

src/MongoDB.Driver.Core/ChangeStreamDocument.cs

@@ -144,6 +144,11 @@ public TDocument FullDocument
                 // if TDocument is BsonDocument avoid deserializing it again to prevent possible duplicate element name errors
                 if (typeof(TDocument) == typeof(BsonDocument) && BackingDocument.TryGetValue("fullDocument", out var fullDocument))
                 {
+                    if (fullDocument.IsBsonNull)
+                    {
+                        return default;
+                    }
+
                     return (TDocument)(object)fullDocument.AsBsonDocument;
                 }
                 else

tests/MongoDB.Driver.Core.Tests/ChangeStreamDocumentTests.cs

@@ -323,10 +323,12 @@ public void FullDocument_should_allow_duplicate_elements()
             secondElement.Value.Should().Be(2);
         }
 
-        [Fact]
-        public void FullDocument_should_return_null_when_not_present()
+        [Theory]
+        [InlineData("{ other : 1 }")]
+        [InlineData("{ other : 1, fullDocument : null }")]
+        public void FullDocument_should_return_null_when_not_present(string changeDocument)
         {
-            var backingDocument = new BsonDocument { { "other", 1 } };
+            var backingDocument = BsonDocument.Parse(changeDocument);
             var subject = CreateSubject(backingDocument: backingDocument);
 
             var result = subject.FullDocument;