fix(editors): add blank entry on Select Editor should happen once

[ghiscoding]

• the problem was noticeable after opening/closing the select dropdown multiple times

[codecov]

Codecov Report

Merging #591 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #591    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          149       149            
  Lines         9917      9917            
  Branches      3499      3353   -146     
==========================================
  Hits          9917      9917            

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

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