API returning invalid pull request activity update item in payload(BBS-75893)

Changing the target branch on a Pull Request generate null commits when making an API PR endpoint call

https://bitbucket.org/api/2.0/repositories/<owner>/<repository>/pullrequests/number/activity

This is caused by commits that became unreachable due to "rebase" away.

If you have an active Pull Request and use rebase away on one of the branches - this might come up so, avoid this may help.