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

[maximilize]

Problem

Document.toString() corrupts an empty block scalar that is followed by a document comment — the comment round-trips back as the scalar's content:

const doc = parseDocument('|5\n#comment')
doc.toJSON() // '' — correct: #comment is at indent 0 < 5, so it's a comment, not content

const out = doc.toString() // '|\n\n\n#comment\n'
parseDocument(out).toJSON() // '\n\n#comment\n'  ← should be ''

Fixes #687.

Cause

blockString() returns a bare |/> for an empty value, dropping the indent indicator even when ctx.forceBlockIndent is set — which stringifyDocument sets precisely when a document comment follows. Without the indicator the empty scalar has no explicit extent, so the trailing comment is absorbed as its content on re-parse.

Fix

Emit the indent indicator for the empty-value case when forceBlockIndent is set, mirroring the existing indentSize logic a few lines below.

Tests

Added a round-trip regression test in tests/doc/comments.ts. The full suite passes, including the YAML and JSON test suites — no regressions.

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🤖 This pull request was written with AI assistance (Claude Code).