Change OSPushSubscriptionState's "nil" json values to be ""

* The dictionary representation of the OSPushSubscriptionState would be better as empty strings instead of the string literal "nil".
* This way, it is easier to check for the emptiness of these properties.

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalUser/Source/OSSubscriptionModel.swift

@@ -53,8 +53,8 @@ public class OSPushSubscriptionState: NSObject {
     }
 
     @objc public func jsonRepresentation() -> NSDictionary {
-        let id = self.id ?? "nil"
-        let token = self.token ?? "nil"
+        let id = self.id ?? ""
+        let token = self.token ?? ""
         return [
             "id": id,
             "token": token,