fix: do not output undefined when sourceRoot is unavailable

[alexander-akait]

This PR contains a:

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

Motivation / Use-Case

fixes #1034

Breaking Changes

No

Additional Info

mozilla/source-map#340

sourceRoot is optional

[codecov]

Codecov Report

Merging #1036 into master will decrease coverage by 2.3%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1036      +/-   ##
==========================================
- Coverage   99.15%   96.84%   -2.31%     
==========================================
  Files          10       10              
  Lines         475      476       +1     
  Branches      145      147       +2     
==========================================
- Hits          471      461      -10     
- Misses          4       12       +8     
- Partials        0        3       +3

Impacted Files	Coverage Δ
src/runtime/api.js	64.7% <100%> (-32.36%)	⬇️
src/utils.js	98.96% <0%> (ø)	⬆️

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