Transform empty-string argument

[PoffM]

Added extra case to convert an empty-string argument to valid RSQL empty string.

Fixes #10

Checklist

• [x] Functionality works & meets specifications
• [x] Code reviewed
• [x] Code unit/integration/system tested
• N/A User documentation updated
• [x] Clean commits
• [x] No warnings during install
• N/A Updated flow typing
• [x] Added Feature/Fix to release notes

[codecov[bot]]

Codecov Report

Merging #11 into master will increase coverage by 1.7%. The diff coverage is 100%.

@@           Coverage Diff            @@
##           master     #11     +/-   ##
========================================
+ Coverage    94.8%   96.6%   +1.7%     
========================================
  Files           1       1             
  Lines          58      59      +1     
  Branches       11      12      +1     
========================================
+ Hits           55      57      +2     
+ Misses          3       2      -1

Flag	Coverage Δ
#unit	96.6% <100%> (+1.7%)	:arrow_up:

Impacted Files	Coverage Δ
src/rsql.js	96.6% <100%> (+1.7%)	:arrow_up:

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[PoffM]

Thanks for the quick response! Though I think your 0.1.2 build/publish failed :(