fix(filters): make sure Select Editor/Filter collection is filled

[ghiscoding]

• add an extra collection length check before rendering the Select Editor/Filter dropdown to avoid multiple-select.js throwing errors that it can't find list or items

[codecov]

Codecov Report

Merging #700 (ad16259) into master (09ac192) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #700   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          159       159           
  Lines        10590     10595    +5     
  Branches      3727      3728    +1     
=========================================
+ Hits         10590     10595    +5     

Impacted Files	Coverage Δ
.../modules/angular-slickgrid/editors/selectEditor.ts	100.00% <100.00%> (ø)
.../modules/angular-slickgrid/filters/selectFilter.ts	100.00% <100.00%> (ø)

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