Adds discussion to the optimistic design response

This prevents a warning from occouring when we upload a design

changelogs/unreleased/33306-missing-field-discussions.yml

+---
+title: Prevents console warning on design upload
+merge_request: 19297
+author:
+type: fixed

ee/app/assets/javascripts/design_management/pages/index.vue

@@ -114,6 +114,10 @@ export default {
           startSha: '',
           headSha: '',
         },
+        discussions: {
+          __typename: 'DesignDiscussion',
+          edges: [],
+        },
         versions: {
           __typename: 'DesignVersionConnection',
           edges: {

ee/spec/frontend/design_management/pages/index_spec.js

@@ -179,6 +179,10 @@ describe('Design management index page', () => {
                       startSha: '',
                       headSha: '',
                     },
+                    discussions: {
+                      __typename: 'DesignDiscussion',
+                      edges: [],
+                    },
                     versions: {
                       __typename: 'DesignVersionConnection',
                       edges: {