Add tests for NullValue not appearing in a oneof at all

csharp/protos/unittest_issues.proto

diff --git a/csharp/protos/unittest_issues.proto b/csharp/protos/unittest_issues.proto
index b602d77..ef40d75 100644
--- a/csharp/protos/unittest_issues.proto
+++ b/csharp/protos/unittest_issues.proto
@@ -147,3 +147,7 @@
     google.protobuf.NullValue null_value = 2;
   }
 }
+
+message NullValueNotInOneof {
+  google.protobuf.NullValue null_value = 2;
+}