Snapshots on days with no change

[mikepurvis]

In the unlikely case that there was no change at all in 24h between snapshots, the git commit would become a no-op with return code 1, which as it stands would fail the whole pipeline. Some options for this scenario:

• || true the return code, meaning no tag will be created and we'll just proceed with tagging the previous commit.
• detect no change and gracefully stop the pipeline without creating a tag at all.
• add --allow-empty so that we get a new commit no matter what.
• let the pipeline just fail on those days.
• break up the pipeline into separate "detect" and "create" portions, where the first only triggers the second if changes actually exist to be captured.

[mikepurvis]

First day of no change occurred: https://github.com/clearpathrobotics/rosdistro-snapshots/actions/runs/3217153659/jobs/5259803775