null values in sparse map serialization and deserialization

[AndrewAsseily]

Backport of aws/aws-cli#10177.

In this PR:

• JSON and CBOR serializers now handle None values in sparse map and list inputs, same pattern that #3618 added for list parsing
• CBOR map parser was silently dropping null values (0xf6) instead of preserving them as None in the resulting dict. This aligns CBOR map parsing with how _parse_array already handles nulls, and with the new JSON parser behavior
• Updated protocol tests

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