Don't remove default table mapping for TVF return entity types.

[AndriySvyryd]

Preserve null mappings in the model snapshot.
Add non-generic ToSqlQuery overload.
Output Fluent API for ToSqlQuery in the model snapshot.
Fix SetFunctionName setting incorrect annotation.

Fixes #23408

[bricelam]

This feels like it's abusing Code.UnknownLiteral(). I wonder if we need a new method like Code.NullableLiteral(value, compilerInfersType: false). But I'm ok with it as is for now.

[ghost]

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[ghost]

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