Check nil on fmt stringer

[kolaworld]

• Prevent panic when calling log.Stringer(nil)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f9148cd) 76.78% compared to head (52c0b8a) 76.83%.

❗ Current head 52c0b8a differs from pull request most recent head 3c23cdb. Consider uploading reports for the commit 3c23cdb to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #333      +/-   ##
==========================================
+ Coverage   76.78%   76.83%   +0.04%     
==========================================
  Files          28       28              
  Lines        1034     1036       +2     
==========================================
+ Hits          794      796       +2     
  Misses        178      178              
  Partials       62       62              

Files	Coverage Δ
log/model_valuer.go	100.00% <100.00%> (ø)

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