Option to hard-fail when current_sha is empty

[beatak]

Feature request

• [x] If the maintainers agree with the feature as described here, I intend to submit a Pull Request myself.
  • I can try, if someone points me how to run a test suite in rancher-desktop instead of minikube

Proposal: Currently, if you do krane --current-sha="" … you'll get current-sha is optional but can not be blank, it seems like a failure, but it still outputs the manifest, and the build could go thru. So, I'd like to propose a way to make it a hard-stop. I'm open to suggestions on whether the flag should be a command line argument like --fail-blank-current-sha, or some environmental variable KRANE_FAIL_BLANK_CURRENT_SHA (I see KRANE_LOG_LINE_LIMIT exists).