Fix intermittent package_versions.py error when retrieving pkg:deb/debian packages

As described in detail in [purldb issue 247](https://github.com/nexB/purldb/issues/247#issuecomment-1977834376), the server for `pkg:deb/debian/*` queries intermittently fails to respond properly and instead returns a status code error (often 503) and a traceback that interrupts the flow of the purlcli command (`versions`). I've seen this behavior off and on in my recent weeks of testing involving deb/debian packages.

[One reply comment](https://github.com/nexB/purldb/issues/247#issuecomment-1978408538) has suggested that the solution may be returning an empty list.