Based on github API documentation: https://docs.github.com/en/rest/pulls/pulls#get-a-pull-request

The value of the mergeable attribute can be true, false, or null. If the value is null, then GitHub has started a background job to compute the mergeability. After giving the job time to complete, resubmit the request. When the job finishes, you will see a non-null value for the mergeable attribute in the response. If mergeable is true, then merge_commit_sha will be the SHA of the test merge commit.

The if condition to test if PR is mergeable was wrong. This PR is to fix it.