fix: don't add trailing newline for empty values with spaceBefore

[harihara-ai]

Fixes #639.

Root cause

When a YAML pair has an empty value followed by blank lines (e.g. b: in a:\n b:\n\nc:), the parser stores those blank lines in the pair's sep tokens. When the nested block-map exits before a new item starts at the same level, the special "move blank lines to next item's start" handling in the parser never fires. As a result, resolveProps processes the blank lines in sep and sets spaceBefore: true on the empty value node.

In stringifyPair, this causes ws = '\n' (from vsb = !!value.spaceBefore), which adds a trailing newline to the pair string ('b:\n'). That trailing newline then combines with the blank line added at the parent collection level via pair.key.spaceBefore for the following pair, producing a double blank line.

Fix

One line in stringifyPair.ts: when ws === '\n' (set from value.spaceBefore with no commentBefore) and valueStr === '' (empty value), set ws = ''. The blank line separation between entries is already handled by the parent collection's pair.key.spaceBefore logic — the trailing newline from vsb is redundant and incorrect for empty values.

Tests

Three test cases added inside the existing No extra whitespace for empty values describe block in tests/doc/stringify.ts, matching the minimal examples from the issue:

parseDocument('a:\n  b:\n\nc:').toString()   // was 'a:\n  b:\n\n\nc:\n' → now 'a:\n  b:\n\nc:\n'
parseDocument('- a:\n\n- b').toString()       // was '- a:\n\n\n- b\n'   → now '- a:\n\n- b\n'
parseDocument('a:\n\nb:').toString()          // was 'a:\n\n\nb:\n'       → now 'a:\n\nb:\n'

[eemeli]

Closing due to undeclared LLM use.