Fix GITHUB_BEFORE_SHA initialization for push events

[ferrarimarco]

Proposed changes

• Validate GITHUB_BEFORE_SHA before using it.
• Fix GITHUB_BEFORE_SHA initialization on push events when pushing the first commits. Fix GITHUB_BEFORE_SHA initialization on push events #5096 fixed this initialization for push events that are not the first.

Readiness checklist

In order to have this pull request merged, complete the following tasks.

Pull request author tasks

• I included all the needed documentation for this change.
• I provided the necessary tests.
• I squashed all the commits into a single commit.
• I followed the Conventional Commit v1.0.0 spec.
• I wrote the necessary upgrade instructions in the upgrade guide.
• If this pull request is about and existing issue,
  I added the Fix #ISSUE_NUMBER label to the description of the pull request.

Super-linter maintainer tasks

• Label as breaking if this change breaks compatibility with the previous released version.
• Label as either: automation, bug, documentation, enhancement, infrastructure.