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.

addons/api/tests/unit/serializers/group-test.js

@@ -45,4 +45,34 @@ module('Unit | Serializer | group', function(hooks) {
     });
   });
 
+  test('it normalizes missing member_ids to empty array', function (assert) {
+    assert.expect(1);
+    const store = this.owner.lookup('service:store');
+    const serializer = store.serializerFor('group');
+    const group = store.createRecord('group').constructor;
+    const payload = {
+      id: '1',
+      name: 'Group 1',
+      scope: { id: 'o_123' }
+    };
+    const normalized = serializer.normalizeSingleResponse(
+      store,
+      group,
+      payload
+    );
+    assert.deepEqual(normalized, {
+      included: [],
+      data: {
+        id: '1',
+        type: 'group',
+        attributes: {
+          name: 'Group 1',
+          scope: { scope_id: 'o_123' },
+          member_ids: []
+        },
+        relationships: {},
+      },
+    });
+  });
+
 });

ui/core/mirage/serializers/group.js

@@ -12,7 +12,7 @@ export default ApplicationSerializer.extend({
 
   _hashForModel(model) {
     const json = ApplicationSerializer.prototype._hashForModel.apply(this, arguments);
-    json.member_ids = model.memberIds;
+    if (model.memberIds.length) json.member_ids = model.memberIds;
     return json;
   }