v4.15.1 - Allow null userID if customID is present

NOTE: the typing has been loosened to allow nullable userID, but if a customID is not present, it will continue to throw at runtime

Also note - the SDK does not validate the ID type of the condition being evaluated. You must pass the proper ID type with the call to checkGate/getConfig/getExperiment/getLayer - we suggest passing whichever IDs are known at the time to every check (e.g. pass the userID and customIDs to every check, and let the SDK evaluate on the proper ID). Be sure to logEvent with both IDs so experiments on each ID type can join metrics with exposure data.