Handle Null type in TypeSignatureClass more consistently

[alan910127]

Is your feature request related to a problem or challenge?

Discovered in #14763 and #14872.

Currently, NativeType::Null is treated as a special case in TypeSignatureClass::matches_native_type. This leads to an error in TypeSignatureClass::default_casted_type when the signature class is not TypeSignatureClass::Native, since both native_type and origin_type end up being Null. This results in an inconsistency between the two functions.

Describe the solution you'd like

One approach is to add TypeSignatureClass::Native(logical_null()) to the allowed_source_types of Coercion::Implicit wherever NULL is valid. However, this would require modifying many places in the codebase.

Describe alternatives you've considered

Another option is introducing a Coercion::new_nullable function, which takes the same arguments as Coercion::new_implicit and automatically adds TypeSignatureClass::Native(logical_null()) to allowed_source_type.

Additional context

Both approaches require significant changes, so I’d love to hear thoughts on which direction makes the most sense.

[alan910127]

take

[alan910127]

Since there are no other ideas, I will go with adding NULL to the allowed source types list wherever possible.