fix: add explicit indent indicator to empty block scalar when followed by a document comment

[naveentehrpariya]

Problem

An empty block scalar whose document has a trailing comment produces a toString() output that does not round-trip correctly.

Reproducer (issue #687):

import { parseDocument } from 'yaml'

const doc = parseDocument('|5\n#comment')
// |5 → literal block scalar, explicit indent 5
// #comment is at indent 0 (< 5) → YAML comment, not block content
console.log(doc.toJSON())       // ''  ✓

const str = doc.toString()
console.log(str)                // '|\n\n\n#comment\n'  ← bug

console.log(parseDocument(str).toJSON())  // '\n\n#comment\n'  ← should be ''

What goes wrong: stringifyDocument sets ctx.forceBlockIndent = true when a document comment exists (so that the comment can be placed after the block scalar). For non-empty block scalars, forceBlockIndent causes a two-space indent in the header, which correctly pushes the comment outside the scalar's content. But the early-return path for empty strings (if (!value) return literal ? '|\n' : '>\n') ignores indent, emitting a bare | with no explicit indent indicator.

After the document comment is appended, the output looks like:

|       ← no indent indicator → content determined by first non-empty line
        ← empty line
        ← empty line
#comment  ← indent 0, becomes the first content line → value is "\n\n#comment\n"

On re-parse, #comment is block-scalar content, not a YAML comment — corrupting the round-trip.

Fix

When indent is non-empty (i.e. forceBlockIndent or a document-marker-safe context requires it), emit |1 / >1 instead of | / > for an empty block scalar. The explicit indent indicator 1 means content must start at indent ≥ 1; the comment at indent 0 therefore falls outside the scalar and is correctly interpreted as a YAML comment on re-parse.

- if (!value) return literal ? '|\n' : '>\n'
+ if (!value) return literal ? (indent ? '|1\n' : '|\n') : (indent ? '>1\n' : '>\n')

Tests

• Regression test added for toString() incorrectly converts comment to string #687
• 244/244 stringify tests pass (up from 243)
• 956/956 total tests pass

[eemeli]

Closing due to undeclared LLM use.