fix crash when checking name but no sourcesContent is provided

[sokra]

fixes #121
fixes webpack/webpack#13879

[codecov]

Codecov Report

Merging #128 (5222eec) into main (cfebf95) will increase coverage by 0.91%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
+ Coverage   92.52%   93.44%   +0.91%     
==========================================
  Files          21       21              
  Lines        1579     1587       +8     
  Branches      461      463       +2     
==========================================
+ Hits         1461     1483      +22     
+ Misses        109       97      -12     
+ Partials        9        7       -2     

Flag	Coverage Δ
integration	93.44% <87.50%> (+0.91%)	⬆️

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Impacted Files	Coverage Δ
lib/helpers/streamChunksOfCombinedSourceMap.js	94.88% <87.50%> (+7.38%)	⬆️
lib/helpers/getFromStreamChunks.js	92.59% <0.00%> (+1.85%)	⬆️
lib/helpers/readMappings.js	100.00% <0.00%> (+1.88%)	⬆️

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