Fix AnnotatedPullRequest inner PR nil for new changesets
Created by: eseliger
When an unpublished changeset is loaded, the inner PullRequest is nil since the metadata is just {}
. This causes a nil panic on apply for a changeset on bitbucket cloud when we update it by reapplying.
See this go playground showing how this works and where it falls apart: https://go.dev/play/p/PrPIQRYSoux.
Test plan
Verified that no panic is observed anymore.