Merge branch '508213_fix_500_error_for_branch_rules' into 'master'

BranchRuleDelete mutation: fix 500 error when rule is missing

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175275



Merged-by: Vasilii Iakliushin <viakliushin@gitlab.com>
Approved-by: Joe Woodward <jwoodward@gitlab.com>
Approved-by: Ethan Urie <eurie@gitlab.com>
Reviewed-by: Vasilii Iakliushin <viakliushin@gitlab.com>
Reviewed-by: Joe Woodward <jwoodward@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>