Skip GH action *plus* tests only when secrets aren't present

[alessfg]

Proposed changes

Update GitHub actions to only skip *plus* scenarios when the NGINX Plus license secrets are not present (it used to only run the NGINX Plus test scenarios during internal PRs).

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have added Molecule tests that prove my fix is effective or that my feature works
• I have checked that any relevant Molecule tests pass after adding my changes
• I have updated any relevant documentation (defaults/main/*.yml, README.md and CHANGELOG.md)