Add a check for invalid HAR files that lack the 'log' root property

[na--]

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[codecov-io]

Codecov Report

Merging #861 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #861      +/-   ##
==========================================
- Coverage   69.98%   69.96%   -0.02%     
==========================================
  Files         111      111              
  Lines        8712     8714       +2     
==========================================
  Hits         6097     6097              
- Misses       2220     2221       +1     
- Partials      395      396       +1

Impacted Files	Coverage Δ
converter/har/converter.go	64.36% <0%> (-0.48%)	⬇️

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[mstoykov]

LGTM