[Qualys] Empty scans rechecked at every execution

[qmontal]

Describe the bug
When the Qualys instance have scans without any vulnerability, it gets skipped; as it gets skipped, no file is being downloaded, and on next run, VulnWhisperer will detect it as new and retry to download it.

Affected module
Qualys Vulnerability Management

To Reproduce
Steps to reproduce the behavior:

1. Qualys VM with scans without vulnerabilities
2. Run VulnWhisperer

Expected behavior
Not retry scans that have already been processed.

System in which VulnWhisperer runs (please complete the following information):

• OS: Ubuntu Server
• Version: 18.04 LTS