fix(priority): don't insert an empty [# ] cookie when clearing priority

[liamrlawrence]

Summary

• Headline:set_priority only treated a blank priority as "remove the cookie"
  when a cookie already existed. The other branches ran when the headline had no
  cookie yet and always built a new one, even for a blank priority. As a result,
  pressing <Space> at the priority prompt on a headline with no existing cookie
  returned " " from PriorityState:prompt_user, fell through to an insertion
  branch, and produced a literal [# ] cookie instead of clearing the priority.

Changes

• Move the empty-priority check to the top of set_priority so it applies to all
  paths: if the trimmed priority is empty, remove the existing cookie if present,
  otherwise do nothing. The insertion branches now only run with a real priority
  and can no longer fabricate an empty [# ] cookie.
• Add a test covering <Space> on a headline without an existing priority.

Checklist

I confirm that I have:

• Followed the
  Conventional Commits
  specification (e.g., feat: add new feature, fix: correct bug,
  docs: update documentation).
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary.
• Thoroughly tested my changes.
• Added tests (if applicable) and verified existing tests pass with
  make test.
• Checked for breaking changes and documented them, if any.