Fix JSON conversion for dict[None, T] keys

[hsusul]

Summary

Normalize the literal None key argument produced by built-in
dict[None, T] type hints so JSON object keys serialized as "null" are
restored to Python None.

Fixes #1237.

Root cause

Built-in generics and typing.Dict expose this annotation differently:

• dict[None, str] yields literal None as its key argument
• typing.Dict[None, str] yields NoneType

The mapping converter treated literal None as an absent key type, so it left
the JSON key as "null".

The normalization is intentionally mapping-specific. It does not alter unrelated
annotations such as Literal[None].

Changes

• normalize literal None to NoneType while selecting a mapping key converter
• update the existing known-bug assertion to expect {None: "1"}
• add an Unreleased changelog entry

Validation

• focused test_json_type_hints
• complete tests/test_converter.py: 28 passed
• repository-wide poe lint
• git diff --check

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