[firebase_analytics] Allow setUserID input to be null

[rockwotj]

See the below reference guide links, setting userID to null is equivalent to signing a user out.

https://firebase.google.com/docs/reference/android/com/google/firebase/analytics/FirebaseAnalytics#setUserId(java.lang.String)

https://firebase.google.com/docs/reference/ios/firebaseanalytics/api/reference/Classes/FIRAnalytics#setuserid

[kroikie]

@rockwotj Thanks for this PR and bringing the plugins closer to the native SDKs. However I think the handling of the user ID on the native side will need to be updated to handle the null case since currently it assumes that the ID coming over the channel is never null.

See the Android implementation.

  private void handleSetUserId(MethodCall call, Result result) {
    final String id = (String) call.arguments;
    firebaseAnalytics.setUserId(id);
    result.success(null);
  }

I think this would break if the user ID is null. Could you update the native handlers so they don't break when null is passed?

[rockwotj]

Hey @kroikie,

I don't think that actually breaks. It's totally valid to case null to String, see: https://repl.it/repls/ReadySleepyAttribute

Additionally the iOS implementation casts nil to NSString* which is also a valid cast.

And the flutter guide says they convert to those types.
https://flutter.io/docs/development/platform-integration/platform-channels

[kroikie]

@rockwotj You are quite correct, thanks for clarifying that.