fix(formatter): exportWithFormatter should work with undefined item prop

[ghiscoding-SE]

• in some cases the user might have a formatter without any value or a "field" that is not in the dataContext, this however shouldn't block the use of exportWithFormatter

[codecov]

Codecov Report

Merging #457 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #457   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          147       147           
  Lines         9203      9204    +1     
  Branches      3058      3059    +1     
=========================================
+ Hits          9203      9204    +1     

Impacted Files	Coverage Δ
...les/angular-slickgrid/services/export-utilities.ts	100.00% <100.00%> (ø)

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