Release notes are the sole information users have about what could
either have improved or have been broken for them. Omitting release
notes is one thing, but not writing any or having empty ones helps
nobody - users get no context on what could cause issues they were
seeing nor do they know about potential improvements, triagers cannot
easily find the PR that could have introduced the bug either.

Thus, this PR now makes it so that danger actually requires one to write
release notes. Empty release notes or the placeholder will no longer be
accepted.

Release Notes:

- N/A