Allow userId to be nullable on identify for iOS

[danielgomezrico]

What does this PR do? Allow that userId to be null on identify

How should this be manually tested? Call identify with userId null, like:

[[SEGAnalytics sharedAnalytics] identify: null traits: {"something": "other"}];

It throws an exception:

[NSNull length]: unrecognized selector sent to instance 0x1db450558

Fix https://github.com/segmentio/analytics-ios/issues/1024

Screenshots or screencasts (if UI/UX change) On docs the userId is defined as optional https://segment.com/docs/connections/sources/catalog/libraries/mobile/ios/#identify

[danielgomezrico]

@prayansh Hi, Im not sure why the CI fails here, it looks like it is a configuration error from the repo, can we merge this? it is pretty important for our company to allow this fast

[danielgomezrico]

@prayansh ping

[HerrNiklasRaab]

Hey @wenxi-zeng, any chance this can get merged real quick?

[danielgomezrico]

👀 ping