Merge branch '383326-mr-conflicts-source-branch-deleted-fix' into 'master'

Fix helper to only list conflicts when source/target branch exists

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/104664



Merged-by: Tetiana Chupryna <tchupryna@gitlab.com>
Approved-by: Sincheol (David) Kim <dkim@gitlab.com>
Approved-by: Tetiana Chupryna <tchupryna@gitlab.com>
Co-authored-by: Patrick Bajao <ebajao@gitlab.com>