Write: fix empty heading and blockquote block markup in convertToBlocks

[kbrown9]

Fixes RSM-2297

Proposed changes

• Strip lone <br> placeholders from innerHTML in convertToBlocks() before the empty-content check — these are left by contentEditable for focus but produce invalid Gutenberg block markup
• Preserve empty headings, blockquotes, and lists as valid empty Gutenberg blocks instead of skipping them, matching how the block editor handles empty content
• Empty paragraphs are still skipped as they are typically whitespace noise from pressing Enter
• Empty lists with no <li> children get a single empty list item to produce valid markup

Related product discussion/links

• RSM-2297

Does this pull request change what data or activity we track or use?

No.

Testing instructions

1. Open a Write post in the editor
2. Type /heading and press Enter to insert an empty heading — do not type any text into it
3. Type /quote and press Enter to insert an empty blockquote — do not type any text into it
4. Add some content in other blocks (e.g. a paragraph with text)
5. Save or publish the post, then open it in the Gutenberg block editor
6. Verify that each empty heading/blockquote appears as a single empty block (not two split blocks)
7. Verify that non-empty headings and blockquotes still convert correctly
8. Verify that empty paragraphs are not carried over

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

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Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

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