feat: support image-set without url

[alexander-akait]

This PR contains a:

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

Motivation / Use-Case

fixes #833

Breaking Changes

No

Additional Info

No

[codecov]

Codecov Report

Merging #879 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #879      +/-   ##
==========================================
+ Coverage   99.46%   99.47%   +0.01%     
==========================================
  Files          10       10              
  Lines         371      381      +10     
  Branches      105      109       +4     
==========================================
+ Hits          369      379      +10     
  Misses          2        2

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	⬆️
src/plugins/postcss-url-parser.js	98.43% <100%> (+0.28%)	⬆️
src/runtime/url-escape.js	100% <100%> (ø)	⬆️

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