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>

services/auth/auth_token.go

@@ -32,7 +32,7 @@ var (
 
 func CheckAuthToken(ctx context.Context, value string) (*auth_model.AuthToken, error) {
 	if len(value) == 0 {
-		return nil, nil
+		return nil, nil //nolint:nilnil // the auth method is not applicable
 	}
 
 	parts := strings.SplitN(value, ":", 2)