Previous fingerprint is null if no action has been run on the commit

[yonitou]

Description of the bug

I noticed yesterday that many of my actions have failed and then made a bunch of tests regarding the behavior of expo/expo-github-action/fingerprint@main action.
It looks like that if a fingerprint has not been saved for the previous-git-commit (doesn't matter if we define it as an input or let it empty), the fingerprint returns null instead of generating a new one.

To Reproduce

• Create a GitHub action that will be run on every new/new commit on pull requests
• Use the expo/expo-github-action/fingerprint@main action to create a fingerprint diff
• The previous-fingerprint will be the one saved from the main branch
• Now, checkout to main branch and push something but cancel the action that will be triggered (or make it fail with an error)
• Checkout back on the branch of the pull request, push something. The previous fingerprint will now be null which results in a diff

Expected behavior

• The previous-fingerprint should be created if null or at least, should use the last commit from a successful workflow

Actual behavior

• It results in a null fingerprint