ci: don't fail nightly if there have been no changes that night

Changes contained in this patch

if there are no changes, instead of failing, we exit with a 0 status code, then create an empty commit stating that fact. otherwise we commit as usual.

This makes it so that nightlies don't appear to have failed if there were just not any new changes merged on the master that night.

MR Checklist

• This MR solves the problem described in the following issue: #27127 (closed)
• A changelog entry was added in changelog.d/ for user-facing changes (see changelog guide). If this MR does not need a changelog entry, the no-changelog label was applied.
• This MR does not make any significant changes to base, or it has an accompanying CLC proposal.
• If this MR has the potential to break user programs, the user-facing label was applied to test against head.hackage.
• All commits are either individually buildable or squashed.
• Commit messages describe what they do, referring to tickets using #NNNNN syntax.
• Source comments describing the change were added. For larger changes notes and cross-references from the relevant places were added (as applicable).
• Testcases to the testsuite were added (as applicable).
• The users guide was updated (as applicable).