fix(editor): shouldn't call cell changed when cell value is undefined

[ghiscoding]

• we were already checking for null value but undefined is another possibility, so now it checks for null/undefined in isValueChanged() method

[codecov]

Codecov Report

Merging #516 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #516    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          148       148            
  Lines         9589      9591     +2     
  Branches      3208      3344   +136     
==========================================
+ Hits          9589      9591     +2     

Impacted Files	Coverage Δ
...es/angular-slickgrid/editors/autoCompleteEditor.ts	100.00% <100.00%> (ø)
...dules/angular-slickgrid/editors/dualInputEditor.ts	100.00% <100.00%> (ø)
...p/modules/angular-slickgrid/editors/floatEditor.ts	100.00% <100.00%> (ø)
...modules/angular-slickgrid/editors/integerEditor.ts	100.00% <100.00%> (ø)
...odules/angular-slickgrid/editors/longTextEditor.ts	100.00% <100.00%> (ø)
...pp/modules/angular-slickgrid/editors/textEditor.ts	100.00% <100.00%> (ø)

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