[Fix] iOS Null PushSubscription Id and Token crash

[shepherd-l]

Description

One Line Summary

Fixes crash on iOS when calling OneSignal.User.PushSubscription.Id or OneSignal.User.PushSubscription.Token when they are null.

Details

Motivation

Fixes iOS crash caused by calling strdup with a null parameter.

Testing

Manual testing

Tested by initializing OneSignal with no wifi connection and calling OneSignal.User.PushSubscription.Id and OneSignal.User.PushSubscription.Token, app build with Unity 2022.3.10f1 with a fresh install of the OneSignal example app on a physical iPhone 12 with iOS 15.5.

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• I have reviewed this PR myself, ensuring it meets each checklist item
  • WIP (Work In Progress) is ok, but explain what is still in progress and what you would like feedback on. Start the PR title with "WIP" to indicate this.

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This change is 

[nan-li]

Nit: I approved the PR but a ternary operator may simplify the code. Anytime there is logic along the lines of if x: return x, else return y is a sign that a ternary operator may suffice.

return condition ? valueIfTrue : valueIfFalse

[jinliu9508]

I think this looks good and easier to read comparing to using ternary operator.