fix: 🐛 handle empty member_ids in group responses

addons/api/addon/models/group.js

@@ -11,7 +11,11 @@ export default class GroupModel extends GeneratedGroupModel {
    * Members is read-only under normal circumstances.  But members can
    * be persisted via calls to `addMembers()` or `removeMembers()`.
    */
-  @attr({readOnly: true, defaultValue: () => A()}) member_ids;
+  @attr({
+    readOnly: true,
+    defaultValue: () => A(),
+    emptyArrayIfMissing: true
+  }) member_ids;
 
   /**
    * An array of resolved user instances.