fix(export): preserve attributes before an empty style attribute (#682)

[tongshu2023]

Bug

formatAttributes in lib/export/html-parser/stringify.ts drops every attribute accumulated before an empty style attribute, because the empty-style branch returns '' — which becomes the new reduce accumulator — instead of the accumulated string. Closes #682.

if (key === 'style' && !value) return '';   // resets the accumulator

For an element like { id: 'box', style: '' }, serialization produced <div> instead of <div id='box'> — the id (and any other earlier attribute) silently disappears from exported HTML.

Fix

Return the accumulator unchanged so the empty style is skipped without discarding prior attributes:

if (key === 'style' && !value) return attrs;

Tests

Adds tests/export/stringify.test.ts (none existed for this module):

• formatAttributes: value attribute quoting, null → boolean attribute, single→double quote switch when the value contains ', empty style omitted, regression: attributes before an empty style are kept, non-empty style preserved
• toHTML: element whose first attribute precedes an empty style (regression), void-tag serialization, text/comment passthrough

Verification

$ npx vitest run tests/export/stringify.test.ts
 Test Files  1 passed (1)
      Tests  9 passed (9)

prettier --check, eslint, and tsc --noEmit all clean.

[tongshu2023]

Apologies — I missed that #683 (opened earlier by the team) already covers this. Closing in favor of that PR. The tests added here are free to cherry-pick if useful.