[bugfix] Do not require handlers$ in the logging config

[vkarak]

This allows config files to define logging partially for specific systems and inherit its value from the builtin config.

[codecov-commenter]

Codecov Report

Base: 86.50% // Head: 86.50% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (94aed99) compared to base (32f9e25).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2692   +/-   ##
=======================================
  Coverage   86.50%   86.50%           
=======================================
  Files          60       60           
  Lines       11123    11125    +2     
=======================================
+ Hits         9622     9624    +2     
  Misses       1501     1501           

Impacted Files	Coverage Δ
reframe/core/config.py	94.58% <100.00%> (+0.02%)	⬆️

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