When sub ID is nullified, don't make server call

* Don't update the subscription ID to the server. Locally, we may set it to null if we get 404s and later we will get a value when user create is called.
* We were sending an Update Subscription request with "subscriptionId" = null which isn't even a valid property anyway and no-ops thus far

iOS_SDK/OneSignalSDK/OneSignalOSCore/Source/OSModel.swift

@@ -52,11 +52,11 @@ open class OSModel: NSObject, NSCoding {
     }
 
     // We can add operation name to this... , such as enum of "updated", "deleted", "added"
-    public func set<T>(property: String, newValue: T) {
+    public func set<T>(property: String, newValue: T, preventServerUpdate: Bool = false) {
         let changeArgs = OSModelChangedArgs(model: self, property: property, newValue: newValue)
 
         changeNotifier.fire { modelChangeHandler in
-            modelChangeHandler.onModelUpdated(args: changeArgs, hydrating: self.hydrating)
+            modelChangeHandler.onModelUpdated(args: changeArgs, hydrating: self.hydrating || preventServerUpdate)
         }
     }
 

iOS_SDK/OneSignalSDK/OneSignalUser/Source/OSSubscriptionModel.swift

@@ -117,13 +117,19 @@ class OSSubscriptionModel: OSModel {
         }
     }
 
-    // Set via server response
+    /**
+     Typically, the subscription ID is set via server response, so don't trigger a server update call when it changes.
+     It can also be set to null by the SDK when the user or subscription is detected as missing.
+     Setting the subscription ID to null will serve as a "reset" and will later hydrate a value from a user create rquest.
+     */
     var subscriptionId: String? {
         didSet {
             guard subscriptionId != oldValue else {
                 return
             }
-            self.set(property: "subscriptionId", newValue: subscriptionId)
+
+            // If the ID has changed, don't trigger a server call, since it can be set to null
+            self.set(property: "subscriptionId", newValue: subscriptionId, preventServerUpdate: true)
 
             guard self.type == .push else {
                 return