Fix designUploadMutation's optimistic response

Adds `errors` and `skippedDesigns` keys to the optimistic response
for designUploadMutation.

This resolved console warnings when uploading a design.

ee/app/assets/javascripts/design_management/utils/design_management_utils.js

@@ -83,6 +83,8 @@ export const designUploadOptimisticResponse = files => {
     designManagementUpload: {
       __typename: 'DesignManagementUploadPayload',
       designs,
+      skippedDesigns: [],
+      errors: [],
     },
   };
 };

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

@@ -218,6 +218,8 @@ describe('Design management index page', () => {
                 },
               },
             ],
+            skippedDesigns: [],
+            errors: [],
           },
         },
       };

ee/spec/frontend/design_management/utils/design_management_utils_spec.js

@@ -98,6 +98,8 @@ describe('optimistic responses', () => {
             },
           },
         ],
+        errors: [],
+        skippedDesigns: [],
       },
     };
     expect(designUploadOptimisticResponse([{ name: 'test' }])).toEqual(expectedResponse);