🐛 Fixed error when creating subscribers via the admin area

refs https://github.com/TryGhost/Ghost/issues/10569
- updates the Subscriber serialiser to strip the `status` property from the API request when saving if it's falsy

ghost/admin/app/serializers/subscriber.js

@@ -5,5 +5,16 @@ export default ApplicationSerializer.extend({
         unsubscribedAtUTC: {key: 'unsubscribed_at'},
         createdAtUTC: {key: 'created_at'},
         updatedAtUTC: {key: 'updated_at'}
+    },
+
+    serialize() {
+        let json = this._super(...arguments);
+
+        // the API can't handle `status` being `null`
+        if (!json.status) {
+            delete json.status;
+        }
+
+        return json;
     }
 });