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

Swallow error just like in #20839, for the case where there is no
protected branch.

Fixes #20826 for me, though I can't tell if this now covers all cases.

services/pull/update.go

@@ -118,6 +118,9 @@ func IsUserAllowedToUpdate(ctx context.Context, pull *issues_model.PullRequest,
 		}
 		prUnit, err := pr.BaseRepo.GetUnit(ctx, 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
 		}