`userId` for Identify should be optional

[danielgomezrico]

The function identify should have the parameter userId as optional.

The main issue with not allowing it is that you can not use the Persona Product nicely, it has an identity resolution based on traits that are sent and with that you don't need to have the unique key id every time on each platform.

TODO

• Allow it to be null on Android (Make userId nullable for Identify on Android #59)
• Allow it to be null on iOS (Identify userId is optional but the implementation does not reflect this segmentio/analytics-ios#1024)
  • Have a temporal fix while the fix is merged on segment side (Temporal code to allow the userId to be null on iOS #112)

Docs

• Documentation for Android SDK:
  

• Documentation for iOS SDK:
  

The same applies for Ruby SDK and others.

Current

userId Is Required

Expected

userId is optional

[danielgomezrico]

On iOS there's a bug where it can not be optional: segmentio/analytics-ios#1024