Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

models/git/protected_branch.go

@@ -318,7 +318,7 @@ func GetProtectedBranchRuleByName(ctx context.Context, repoID int64, ruleName st
 	if err != nil {
 		return nil, err
 	} else if !exist {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 	}
 	return rel, nil
 }
@@ -329,7 +329,7 @@ func GetProtectedBranchRuleByID(ctx context.Context, repoID, ruleID int64) (*Pro
 	if err != nil {
 		return nil, err
 	} else if !exist {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 	}
 	return rel, nil
 }