Preserve explicit null hash fields

[OskarEichler]

Summary

Use hash key ownership, rather than value truthiness, before falling back from string keys to symbol keys.

Reproduction

For {"foo" => nil, :foo => "fallback"}, querying foo currently returns the symbol value even though the canonical string field is explicitly null. Both single and optimized chained fields now preserve the string-key null while symbol-only hashes remain supported.

Verification

• isolated branch: 1,790 examples, zero failures
• three focused single/chained/symbol-fallback checks
• cumulative candidate: 892 current compliance checks, zero failures
• all runtime syntax and Rails 8.1.3.1 integration pass

Compatibility

Symbol fallback remains unchanged when no string key exists. Prepared with AI-assisted source review; no repository tests were changed.