Fix baseline script if no nullable header

[martincostello]

Fix baseline files not being cleared if the #nullable header is not present in the Unshipped file.

Found in domaindrivendev/Swashbuckle.AspNetCore#3317 when I copied the script to use there.

This use case doesn't affect us, but it's good to make it Technically Correct™️.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.23%. Comparing base (1b7defa) to head (3cd350f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2554   +/-   ##
=======================================
  Coverage   96.23%   96.23%           
=======================================
  Files         311      311           
  Lines        7327     7327           
  Branches     1013     1013           
=======================================
  Hits         7051     7051           
  Misses        222      222           
  Partials       54       54           

Flag	Coverage Δ
linux	96.23% <ø> (ø)
macos	96.23% <ø> (ø)
windows	96.22% <ø> (ø)

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