Fix csharp fieldmasktree merge null checking (#12737)

Fixes #12685 for CSharp, but the Java equivalent probably has the same bug.

Closes #12737

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/12737 from marner2:fix_csharp_fieldmasktree_merge_null_checking 78c7812ac3
PiperOrigin-RevId: 579314424

csharp/src/Google.Protobuf.Test/FieldMaskTreeTest.cs

@@ -406,6 +406,12 @@ namespace Google.Protobuf
             Merge(new FieldMaskTree().AddFieldPath("payload.single_int32"),
                 sourceWithPayloadInt32Unset, destination, options, useDynamicMessage);
             Assert.IsNotNull(destination.Payload);
+
+            // Clear unset primitive fields even if source payload is cleared
+            destination = source.Clone();
+            Merge(new FieldMaskTree().AddFieldPath("payload.single_int32"),
+                clearedSource, destination, options, useDynamicMessage);
+            Assert.AreEqual(0, destination.Payload.SingleInt32);
         }
 
         [Test]

csharp/src/Google.Protobuf/FieldMaskTree.cs

@@ -270,6 +270,13 @@ namespace Google.Protobuf
                         field.Accessor.SetValue(destination, destinationField);
                     }
 
+                    if (sourceField == null)
+                    {
+                        // If the message field is not present in the source but is in the destination, create an empty one
+                        // so we can properly handle child entries
+                        sourceField = field.MessageType.Parser.CreateTemplate();
+                    }
+
                     var childPath = path.Length == 0 ? entry.Key : path + "." + entry.Key;
                     Merge(entry.Value, childPath, (IMessage)sourceField, (IMessage)destinationField, options);
                     continue;