make user id nullable in line with java interface

Author: Matthew Ault

packages/core/android/src/main/java/com/segment/analytics/reactnative/core/RNAnalyticsModule.kt

@@ -193,7 +193,7 @@ class RNAnalyticsModule(context: ReactApplicationContext): ReactContextBaseJavaM
 
     @ReactMethod
     fun identify(userId: String, traits: ReadableMap, integrations: ReadableMap, context: ReadableMap) =
-        analytics.identify(userId, Traits() from traits, this.getOptions(integrations))
+        analytics.identify(userId?, Traits() from traits, this.getOptions(integrations))
 
     @ReactMethod
     fun group(groupId: String, traits: ReadableMap, integrations: ReadableMap, context: ReadableMap) =