fix(filters): skipCompoundOperatorFilterWithNullInput skip empty string

[ghiscoding]

• when enabling skipCompoundOperatorFilterWithNullInput it was only working the first time and if the user clicked on the input (or type something and then erases it), it no longer undefined and was no longer skipping null input like it should. So we simply need to compare against not just undefined but also against empty string '' and if so then skip it filtering
• note however that we should only skip null value when previous value was not null so that we get full list back

below is a demo of the small bug

after fix

[stackblitz]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.8%. Comparing base (e942539) to head (84fbd1b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1566     +/-   ##
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+ Coverage    99.8%   99.8%   +0.1%     
========================================
  Files         198     198             
  Lines       21727   21729      +2     
  Branches     7273    7274      +1     
========================================
+ Hits        21666   21668      +2     
  Misses         55      55             
  Partials        6       6             

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