permissions: Allow unsetting permissions model appId

This is needed  due to the fact that Flatseal can switch
back and forw between having applications and not having
applications.

src/models/permissions.js

@@ -254,6 +254,9 @@ var FlatpakPermissionsModel = GObject.registerClass({
     }
 
     _processPendingUpdates() {
+        if (this._appId === '')
+            return;
+
         if (this._delayedHandlerId === 0)
             return;
 
@@ -278,6 +281,9 @@ var FlatpakPermissionsModel = GObject.registerClass({
     }
 
     _setup() {
+        if (this._appId === '')
+            return;
+
         Object.values(MODELS).forEach(model => model.reset());
         this._loadPermissions();
         this._loadGlobalOverrides();