`wbt_wd()`: fix behavior and message for `wd=""`

[brownag]

• RE: #108
  • On fresh install, pre-existing settings.json working_directory in the new install directory is set to ""
  • wbt_wd("") now properly resets the working_directory setting and --wd flag under various conditions

[codecov-commenter]

Codecov Report

Merging #109 (cd36d90) into master (aa9a2b9) will decrease coverage by 0.01%. The diff coverage is 50.00%.

:exclamation: Current head cd36d90 differs from pull request most recent head 0621cc2. Consider uploading reports for the commit 0621cc2 to get more accurate results

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@@            Coverage Diff            @@
##           master    #109      +/-   ##
=========================================
- Coverage    8.87%   8.87%   -0.01%     
=========================================
  Files          21      21              
  Lines        8370    8372       +2     
=========================================
  Hits          743     743              
- Misses       7627    7629       +2     

Impacted Files	Coverage Δ
R/wbt.R	56.49% <50.00%> (-0.28%)	:arrow_down:

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