Fixed a bug when using WSL when all Windows paths have been removed

[tleonhardt]

Fixed a bug when using WSL when all Windows paths have been removed from $PATH.

This is a known Pyperclip Issue which ideally should be fixed in Pyperclip. That being said, maintenance on Pyperclip can be a bit slow. So we will go ahead and kludge around this issue in cmd2 in order to provide the best possible experience for our users.

Closes #760

[codecov]

Codecov Report

Merging #761 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #761      +/-   ##
=========================================
+ Coverage    97.3%   97.3%   +<.01%     
=========================================
  Files          13      13              
  Lines        3450    3457       +7     
=========================================
+ Hits         3357    3364       +7     
  Misses         93      93

Impacted Files	Coverage Δ
cmd2/clipboard.py	100% <100%> (ø)	⬆️
cmd2/parsing.py	100% <0%> (ø)	⬆️
cmd2/plugin.py	100% <0%> (ø)	⬆️
cmd2/history.py	100% <0%> (ø)	⬆️

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