perf(sourcemaps): add back lazy sourcemap decode and handling nullish mappings

[thebanjomatic]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

resolves #5084

Description

We weren't properly handling ill-formed (according to types) sourcemap input such as {mappings: null} {mappings: undefined} or {}. This PR expands the set of values that can be passed in to decodedSourcemaps to ensure that we are handling all those edge cases and doing something sane. In this case, we treat mappings: null the same as mappings: '' and return the "empty sourcemap"

return {
  mappings: [],
  names: [],
  sources: [],
  version: 3
};

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
rollup	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 8, 2023 5:08pm

[codecov]

Codecov Report

Merging #5087 (80b2d26) into master (6787f44) will decrease coverage by 0.01%.
Report is 6 commits behind head on master.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master    #5087      +/-   ##
==========================================
- Coverage   98.97%   98.96%   -0.01%     
==========================================
  Files         226      226              
  Lines        8362     8390      +28     
  Branches     2294     2303       +9     
==========================================
+ Hits         8276     8303      +27     
  Misses         31       31              
- Partials       55       56       +1     

Files Changed	Coverage Δ
src/utils/decodedSourcemap.ts	93.54% <92.59%> (+7.83%)	⬆️
src/Module.ts	100.00% <100.00%> (ø)
src/utils/collapseSourcemaps.ts	90.62% <100.00%> (+0.19%)	⬆️
src/utils/getOriginalLocation.ts	100.00% <100.00%> (ø)

[thebanjomatic]

@lukastaegert Sorry to ping you, but can this be merged and re-released if the change looks good? Bug fix is in the second commit to make it clearer what the problem was, but feel free to squash and merge if you'd like.

[lukastaegert]

Ah sorry, thanks for reminding me, I will take care.

[rollup-bot]

This PR has been released as part of rollup@3.28.0. You can test it via npm install rollup.