fix: should generate last column mapping even when source is missing

[hai-x]

Fixes webpack/terser-webpack-plugin#589.

We should generate the column mapping that points to no source(i.e. sourceIndex < 0) in ConcatSource.

We will validate it in createFullMappingsSerializer to ensure it is valid when followed by an active mapping.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.24%. Comparing base (6af577c) to head (0b913d7).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #171   +/-   ##
=======================================
  Coverage   93.24%   93.24%           
=======================================
  Files          25       25           
  Lines        1866     1866           
  Branches      595      596    +1     
=======================================
  Hits         1740     1740           
  Misses        117      117           
  Partials        9        9           

Flag	Coverage Δ
integration	93.24% <100.00%> (ø)

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[alexander-akait]

Looks good, let's fix linting and we can merge