ZAP Backup File Disclosure false positives on HTTP 400 responses

## The problem

ZAP raised Backup File Disclosure alerts on responses that were plain HTTP 400s with empty bodies, which are obviously false positives. The backup file scan rule was matching on responses it should have ignored.

## The fix

Fixed in version 24 of the Active scanner rules (beta) add-on via zaproxy/zap-extensions. Update that add-on and the false positives go away. The reporter confirmed the original issue was resolved.