Release 5.1.2

[nan-li]

What's Changed

⚠️ Behavior Changes

• When you call the jsonRepresentation method of OSPushSubscriptionState, if a property is null, the value will now be "" instead of the string literal "nil" #1373

🐛 Bug Fixes & Misc Improvements

• Rename internal method that was called setSharedInstance to workaround false App Store flagging #1375
• Improve Swift concurrency safety to address crash reports in production #1376
• For our server: Add OneSignal-Subscription-Id to Create User request which improves last_active data #1372
  

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This change is 

[nan-li]

Our released version 5.1.1 causes a build error for users.
This is the same codebase as v5.1.1

[nan-li]

5.1.2 also caused the same issues, it was released manually but the problem was not in the release process.