fix: keep indentation indicator on empty block scalar before a comment

[abhijeet117]

Summary

For an empty block scalar followed by a document-level comment, toString() emitted a bare | header without an indentation indicator. When that output is re-parsed, the comment line is no longer outside the block scalar and becomes part of the scalar value, so parseDocument('|5\n#comment\n').toString() does not round-trip: the comment turns into string data. The empty-value branch now emits an indentation indicator when a top-level comment follows, matching the protection that non-empty block scalars get through forced content indentation.

Testing

Reproduced with parseDocument('|5\n#comment\n'): the first toString() output reparsed as the value "\n\n#comment\n" with no comment. Added a round-trip regression test in tests/doc/comments.ts that fails without this change. Full vitest suite passes (3387 tests), eslint and tsc are clean.

Checklist

• bug reproduced before fix
• root cause identified
• bug fixed
• tests passed