Json representation of User State uses empty string, not "nil"

When giving a jsonRepresentation of OSUserState, choose empty string as the representation of a null value instead of the string literal "nil". The string "nil" can be interpreted as an existing value whereas the empty string is a better option as it can be checked for `isEmpty`.

iOS_SDK/OneSignalSDK/OneSignalUser/Source/OSUserState.swift

@@ -41,8 +41,8 @@ public class OSUserState: NSObject {
 
     @objc public func jsonRepresentation() -> NSDictionary {
         return [
-            "onesignalId": onesignalId ?? "nil",
-            "externalId": externalId ?? "nil"
+            "onesignalId": onesignalId ?? "",
+            "externalId": externalId ?? ""
         ]
     }
 }