Fix 500 error viewing pull request when fork has pull requests disabled (#22512) (#22515)

Backport #22512

Co-authored-by: Brecht Van Lommel <[email protected]>

Author: jolheiser

services/pull/update.go

@@ -109,6 +109,9 @@ func IsUserAllowedToUpdate(ctx context.Context, pull *issues_model.PullRequest,
 	if pr.ProtectedBranch == nil {
 		prUnit, err := pr.BaseRepo.GetUnit(unit.TypePullRequests)
 		if err != nil {
+			if repo_model.IsErrUnitTypeNotExist(err) {
+				return false, false, nil
+			}
 			log.Error("pr.BaseRepo.GetUnit(unit.TypePullRequests): %v", err)
 			return false, false, err
 		}