chore: Replace getWithDefault with nullish coalescing

[iamareebjamal]

Deprecated https://deprecations.emberjs.com/v3.x/#toc_ember-metal-get-with-default

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[niranjan94]

Here is an overview of what got changed by this pull request:

Complexity increasing per file
==============================
- app/router.js  1
         

See the complete overview on Codacy

[codecov]

Codecov Report

Merging #5205 into development will decrease coverage by 0.05%.
The diff coverage is 25.00%.

@@               Coverage Diff               @@
##           development    #5205      +/-   ##
===============================================
- Coverage        22.68%   22.63%   -0.06%     
===============================================
  Files              489      490       +1     
  Lines             5198     5201       +3     
  Branches            36       36              
===============================================
- Hits              1179     1177       -2     
- Misses            4014     4019       +5     
  Partials             5        5              

Impacted Files	Coverage Δ
app/components/modals/cropper-modal.js	0.00% <0.00%> (ø)
app/components/orders/order-summary.js	29.41% <0.00%> (ø)
app/controllers/events/view/tickets/add-order.js	0.00% <0.00%> (ø)
app/router.js	100.00% <100.00%> (ø)
app/components/tabbed-navigation.js	26.66% <0.00%> (-20.01%)	⬇️
app/routes/event-invoice/review.js	0.00% <0.00%> (ø)
app/components/ui-table/cell/cell-date.js	100.00% <0.00%> (ø)

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