[NT-1779] Fix for physical/digital add-ons

[singhhari]

📲 What

In our RewardAddOnSelectionViewModel we are currently getting the value of a reference to shippingRuleExpanded from the first add on returned from the rewards data. This resulted in physical add ons not being shown for Rewards with a digital add on as the first node returned in the response. This PR updates the logic for determining the correct shipping rule to use.

🤔 Why

A creator flagged this a couple of weeks ago and, after identifying the issue, only required a fairly minor tweak.

[jgsamudio]

Looks good! Waiting on the snapshots to match the original

[codecov]

Codecov Report

Merging #1377 (54370b3) into master (5fdbf1c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1377   +/-   ##
=======================================
  Coverage   85.85%   85.85%           
=======================================
  Files        1103     1103           
  Lines       96062    96066    +4     
=======================================
+ Hits        82473    82477    +4     
  Misses      13589    13589           

Impacted Files	Coverage Δ
...lers/RewardAddOnSelectionViewControllerTests.swift	100.00% <100.00%> (ø)
...ary/ViewModels/RewardAddOnSelectionViewModel.swift	98.14% <100.00%> (+0.02%)	⬆️

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