Fix a panic in HAR conversion due to missing length check

[na--]

Edit: the root cause of reaching the state with 0 entries was simply using something invalid with the --only flag in k6 convert like the URL instead of the domain name

[codecov-io]

Codecov Report

Merging #760 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #760      +/-   ##
==========================================
- Coverage   63.91%   63.89%   -0.02%     
==========================================
  Files         102      102              
  Lines        8479     8481       +2     
==========================================
  Hits         5419     5419              
- Misses       2699     2701       +2     
  Partials      361      361

Impacted Files	Coverage Δ
converter/har/converter.go	17.42% <0%> (-0.14%)	⬇️

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