Add authentication check

[MrChrisW]

This adds an authentication check before attempting to create a pull request via the action. If the check fails we do not proceed to create a pull request. It provides a useful error message when reviewing the action logs.

When running this action I noticed the following error when a GITHUB_TOKEN expires:

jq: error (at <stdin>:4): Cannot index string with string "head"

This is caused by an attempt to get the head ref when the API responds with an error:

{ "message": "Bad credentials", "documentation_url": "https://docs.github.com/rest" }

github-action/entrypoint.sh

Line 100 in ec22a49

PULL_REQUESTS=$(echo "$(curl -sSL -H "${AUTH_HEADER}" -H "${HEADER}" -X GET "${PULLS_URL}${PULL_REQUESTS_QUERY_PARAMS}")" | jq --raw-output '.[] | .head.ref ')

[andrii-bodnar]

@MrChrisW thanks a lot for your contribution! 🚀