`SingleOrDefault<(int, int)?>(query)` always returned null because the IsValueTuple did not handle nullable types and thus the `GenerateValueTupleDeserializer` method was never called.

I've added test for the case when the query does not return something (the tuple should be null) and when it does return something. Additionally added tests for the edge-case where you select 8 or 15 element tuples to make sure nothing broke there.