fix: encode sourcesContent null as raw null instead of quoted string

[Boshen]

Summary

• None entries in source_contents were passed as the string "null" through escape_into, producing "null" (a JSON string) instead of the raw JSON null token.
• Write null bytes directly for None entries, only running escape_into for Some values.

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will not alter performance

✅ 13 untouched benchmarks
⏩ 5 skipped benchmarks¹

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_{Comparing fix/sourcecontent-null-encoding (bb6c05a) with main (b45a772)}

Footnotes

1. 5 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩