Fix logging to handler config without loggers

[mraspaud]

This PR allows trollflow2 to accept log configs without loggers.

• Closes Valid log config fails in trollflow2 #197
• Closes trollflow2 fails with KeyError when trying to process a message #192
• Closes Subprocess crash when using custom log config #183
• Tests added
• Tests passed
• Passes flake8 trollflow2

[pnuu]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0713a86) 95.76% compared to head (3bed599) 95.80%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #199      +/-   ##
==========================================
+ Coverage   95.76%   95.80%   +0.03%     
==========================================
  Files          13       13              
  Lines        2972     3000      +28     
==========================================
+ Hits         2846     2874      +28     
  Misses        126      126              

Flag	Coverage Δ
unittests	95.80% <100.00%> (+0.03%)	⬆️

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