Our SQL Server provider seems to always create non-nullable columns, regardless of the .NET type, whereas our PG provider varies based on whether the .NET type is nullable. Do a pass over the relational providers to ensure everything makes sense. I'm not sure any non-relational provider allows defining the nullability of properties/columns, but worth checking to make sure (Azure AI Search seems to maybe have a concept of nullability in its schema).

We may also want to introduce a way for the user to control the column nullability regardless of the .NET type used, that would likely require #10359.