feat(filters): add option to filter empty values for select filter

[ghiscoding]

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[codecov]

Codecov Report

Merging #731 (7c9ce5a) into master (d012a1f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #731   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          159       159           
  Lines        10741     10747    +6     
  Branches      3803      3808    +5     
=========================================
+ Hits         10741     10747    +6     

Impacted Files	Coverage Δ
.../modules/angular-slickgrid/filters/selectFilter.ts	100.00% <100.00%> (ø)
...dules/angular-slickgrid/services/filter.service.ts	100.00% <100.00%> (ø)

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