fix(exports): when cell value is empty object return empty string

[ghiscoding]

• when dealing with complex object, it could in some cases return an empty object when targeted object is undefined or not part of the object itself

[codecov]

Codecov Report

Merging #645 (0534d13) into master (cf69f5d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #645   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          154       154           
  Lines        10369     10374    +5     
  Branches      3652      3654    +2     
=========================================
+ Hits         10369     10374    +5     

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

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