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/issue/assignee.go

@@ -234,7 +234,7 @@ func TeamReviewRequest(ctx context.Context, issue *issues_model.Issue, doer *use
 	}
 
 	if comment == nil || !isAdd {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil because no comment was created or it is a removal
 	}
 
 	return comment, teamReviewRequestNotify(ctx, issue, doer, reviewer, isAdd, comment)