Empty signature should not be return throw a not found exception instead

* Patch handles empty signature

chris burge → made their first commit to this issue’s fork.

Opened an MR with patch 3383742-handle-empty-signature-1.patch.

Manual testing is successful:

1. Ensure no signature is stored in config.
2. Load /.well-known/security.txt.sig
3. Observe a 200 response with an empty body.
4. Deploy merge request.
5. Load /.well-known/security.txt.sig
6. Observe a 400 response with the standard Page Not Found page.