Check for empty vts preferences list

[jjnicola]

What:
Check for empty vts preferences list before trying to add an empty list into redis.

Why:
Adding an empty list is not allowed and raises an exception. Not it is check. This can happen for small scan configs with less VTs without preferences.

How:
I find this issue running a small scan config with only one selected VT "ssh_authorization.nasl"

Checklist:

• Tests
• CHANGELOG Entry

[codecov]

Codecov Report

Merging #340 into ospd-openvas-20.08 will decrease coverage by 0.87%.
The diff coverage is n/a.

@@                  Coverage Diff                   @@
##           ospd-openvas-20.08     #340      +/-   ##
======================================================
- Coverage               82.71%   81.84%   -0.88%     
======================================================
  Files                       9        9              
  Lines                    1481     1520      +39     
======================================================
+ Hits                     1225     1244      +19     
- Misses                    256      276      +20     

Impacted Files	Coverage Δ
ospd_openvas/__version__.py	100.00% <ø> (ø)
ospd_openvas/daemon.py	62.01% <ø> (-0.90%)	⬇️
ospd_openvas/db.py	96.24% <ø> (-3.36%)	⬇️
ospd_openvas/lock.py	85.96% <ø> (+0.25%)	⬆️
ospd_openvas/nvticache.py	98.49% <ø> (+0.01%)	⬆️
ospd_openvas/openvas.py	100.00% <ø> (ø)
ospd_openvas/preferencehandler.py	87.93% <ø> (+0.82%)	⬆️
ospd_openvas/vthelper.py	86.61% <ø> (-2.10%)	⬇️
... and 5 more

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