ci: skip empty commits in deployment-notifier
Created by: jhchabran
In some cases, it's possible that the deploy-sourcegraph pipelines will build an empty commit, ex: https://github.com/sourcegraph/deploy-sourcegraph-cloud/commit/3debd2abc227fe0b0800d02ee2ddebd069c0cc82
This causes the deployment notifier to fail unexpectedly. Being a soft failure, it does not interrupt the build, though it appears as a failure whereas it's not.
Test plan
Manually reproduced locally with an empty commit and qa'ed against it.