fix: crash on modified AST from postcss-loader

[alexander-akait]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

fixes #1267

Breaking Changes

No

Additional Info

No

[codecov]

Codecov Report

Merging #1268 (6f39332) into master (f1e0f00) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1268   +/-   ##
=======================================
  Coverage   99.46%   99.46%           
=======================================
  Files          11       11           
  Lines         742      747    +5     
  Branches      248      253    +5     
=======================================
+ Hits          738      743    +5     
  Misses          4        4           

Impacted Files	Coverage Δ
src/plugins/postcss-import-parser.js	100.00% <100.00%> (ø)
src/plugins/postcss-url-parser.js	100.00% <100.00%> (ø)

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