Translation for `Nullable<T>.ToString()`

[ranma42]

What is the intended translation for Nullable<T>.ToString()?

It is currently unclear (at least to me) what is the intended result (and/or translation) because:

• the C# documentation states that it returns

  The text representation of the value of the current Nullable object if the HasValue property is true, or an empty string ("") if the HasValue property is false.

• EFCore is attempting to test that the translation matches the C# definition in ToString_boolean_property_nullable, but it does so without ever populating the column with NULL
• EFCore returns NULL for NULLs in bool? columns in SQL Server and Sqlite
• EFCore returns the "True" string for NULL in other bool? expressions
  reported as Inconsistent results for Nullable<bool>.ToString() #33941
• EFCore returns "" for enums
• EFCore converts (hence propagates NULL) for several types in SQL Server and Sqlite

I realized that the situation was unclear while working on #33940

[roji]

@ranma42 thanks for digging into this, the current situation indeed seems quite problematic.

Without going into specifics, we'd ideally align the translation with the .NET behavior, which as you say is to return an empty string; this would be a small breaking change, but one which I think would be accepted (as the current behavior is simply buggy). I'll bring this up in a design discussion to make sure the team agrees.

[roji]

Design discussion: we agree that we should align the Nullable ToString() behavior to the .NET one in all cases (empty string); although this is a breaking change, we consider it a minor one, and the current behavior is buggy.

[ranma42]

This should now be fixed in the codebase. Given that it is a breaking change, should it be mentioned in https://github.com/dotnet/EntityFramework.Docs/blob/main/entity-framework/core/what-is-new/ef-core-9.0/breaking-changes.md before closing?