refactor!: consistent null handling in coercible signatures

[alan910127]

Which issue does this PR close?

• Closes #15013.

Rationale for this change

Currently, there is a special case for null handling in TypeSignatureClass::matches_native_type, which causes inconsistencies and issues between TypeSignatureClass::matches_native_type and TypeSignatureClass::default_casted_type (it will hit the else match arm which returns an error) when self is not a TypeSignatureClass::Native.

What changes are included in this PR?

1. Remove the special handling of null case in TypeSignatureClass::matches_native_type
2. Add TypeSignatureClass::Native(logical_null()) where the argument is nullable and it was not originally supplied.

Are these changes tested?

These changes should be covered in existing test.

Are there any user-facing changes?

Yes, if the user rely on the special null case handling behavior of 'TypeSignatureClass::Native, they will need to transform the usage to Coercible::new_implicitand add aTypeSignatureClass::Native(logical_null())`.

[alan910127]

Hi @alamb, I’ve updated upgrading.md. Since this is my first PR with an API change, I’m not 100% sure I did everything right. Would appreciate it if you could take a look and let me know if anything needs adjusting. Thanks!

[alamb]

Thanks @alan910127 -- I'll check it out in a few days

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