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)

[codecov]

Codecov Report

Merging #683 (1a248b0) into master (b2b963c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #683   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          157       157           
  Lines        10451     10451           
  Branches      3669      3669           
=========================================
  Hits         10451     10451           

Impacted Files	Coverage Δ
.../angular-slickgrid/services/excelExport.service.ts	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b2b963c...1a248b0. Read the comment docs.