Don't forward merge ref if it's not up to date

[ofalvai]

New Pull Request Checklist

• Run go fmt on your files (e.g. go fmt ./service/common.go, or on the whole service folder: go fmt ./service/...)
• Write tests for your code
  • The tests should cover both "success" and "error" cases.
  • The tests should also check all the returned variables, don't ignore any returned value!
  • Ideally the tests should be easily readable, we usually use tests to document our code instead of code comments.
    An example, if you'd write a comment like "Given X this function will return Y" or
    "Beware, if the input is X this function will return Y" then you should implement this as
    a unit test, instead of writing it as a comment.
• If your Pull Request is more than a bug fix you should also check README.md and change/add the descriptions there - also
  feel free to add yourself as a contributor if you implement support for a new service ;)
• Before creating the Pull Request you should also run bitrise run test with the Bitrise CLI,
  to perform all the automatic checks (which will run on your Pull Request when you open it).

Summary of Pull Request

The mergeable field can be true, false or null. When it's null, this means the mergeability check is still pending, so using the merge ref in checkouts leads to checking out an outdated state.

This PR changes how we create the build params, we should only include the merge ref if we know it's up to date.

GitHub API: https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#get-a-pull-request