fix(exports): Excel Export custom width applies the width to next column

[ghiscoding]

• the custom width was not applied to the correct column, when user had a custom width on column 1 it was applied to column 2. The bug was that getGrouping() always return an array and we should check for its length instead (not just if it exist)
• fixes issue report in Aurelia-Slickgrid Export column width sets on wrong column, when export to excel aurelia-slickgrid#511

[codecov]

Codecov Report

Merging #242 (cfc7c1f) into master (b5babc3) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #242   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          207       207           
  Lines        11974     11974           
  Branches      4117      4114    -3     
=========================================
  Hits         11974     11974           

Impacted Files	Coverage Δ
packages/excel-export/src/excelExport.service.ts	100.00% <100.00%> (ø)

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