fix: support no assetFileNames configuration

[tivac]

Fixes #447

[codecov]

Codecov Report

Merging #448 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
+ Coverage   99.11%   99.11%   +<.01%     
==========================================
  Files          31       31              
  Lines         789      790       +1     
  Branches      121      121              
==========================================
+ Hits          782      783       +1     
  Misses          7        7

Impacted Files	Coverage Δ
packages/rollup/rollup.js	100% <100%> (ø)	⬆️

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