Check for null before calling strdup

Fixes crash on iOS when calling OneSignal.User.PushSubscription.Id or OneSignal.User.PushSubscription.Token when vales are null. strdup only accounts for string parameters and not null

com.onesignal.unity.ios/Runtime/Plugins/iOS/OneSignalUnityBridgeUser.mm

@@ -161,10 +161,16 @@ void _oneSignalUserRemoveTags(const char* tagsJson) {
     }
 
     const char* _oneSignalPushSubscriptionGetId() {
+        if (OneSignal.User.pushSubscription.id == NULL) {
+            return NULL;
+        }
         return strdup([OneSignal.User.pushSubscription.id UTF8String]);
     }
 
     const char* _oneSignalPushSubscriptionGetToken() {
+        if (OneSignal.User.pushSubscription.token == NULL) {
+            return NULL;
+        }
         return strdup([OneSignal.User.pushSubscription.token UTF8String]);
     }