Fail-safe with empty branch name

[omus]

Branch name on main is resolving to false: https://github.com/beacon-biosignals/docker-build/actions/runs/12876123486/job/35898954790#step:5:262. This change should at least result in not writing out cache-branch-false. Not sure why we aren't picking up main yet.

[omus]

I've ran into an issue. The push event doesn't have a base image to pull from for the initial build on a PR so we always see a failure with the first PR commit for the push event when building off of the cache. There's also an issue on main where it seems the branch is not being picked up properly

[omus]

Ah, part of the problem is using separate image repositories. We aren't really testing a build from a cache for the first commit on a PR as the base SHA being referenced is built in whalesay-push and not whalesay-pr.

[omus]

As it turns out the only way to truly test this is to merge this into main and make another PR to validate it's working I'll go ahead and just do that