Let groovy know whether github hasn't checked the mergeability of a pull request

[LaszloAshin]

The mergeable flag can be null while github hasn't yet decided if the PR is mergeable or not.

See https://github.com/eclipse-egit/egit-github/blob/a30aa361f65d5ccbaca1aedd12651ff2636a3ed9/org.eclipse.egit.github.core/src/org/eclipse/egit/github/core/PullRequest.java#L98-L99
See https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#get-a-pull-request

Testing done

I tested this manually on a local jenkins instance. I couldn't make github return a response with missing mergeable flag but I made sure that my change doesn't break the normal workflow when the flag is present with a value of true or false.

I also added an integration test that runs automatically.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue