Wrap ABSPATH existence short-circuit with code coverage ignore comments

[westonruter]

This addresses the first point of #1789: "Ignore Coverage for Non-Critical Code Blocks"

[github-actions]

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>

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[felixarntz]

Thanks @westonruter!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.58%. Comparing base (c270c43) to head (95884d6).
Report is 2 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #1815      +/-   ##
==========================================
+ Coverage   58.26%   59.58%   +1.32%     
==========================================
  Files          85       84       -1     
  Lines        6670     6522     -148     
==========================================
  Hits         3886     3886              
+ Misses       2784     2636     -148     

Flag	Coverage Δ
multisite	59.58% <ø> (+1.32%)	⬆️
single	36.09% <ø> (+0.83%)	⬆️

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