Conditionally render MR throughput milestone

Fix a bug where the MR Analytics Throughput table
renders column data which does not exist, resulting
in a console error.

ee/app/assets/javascripts/analytics/merge_request_analytics/components/throughput_table.vue

@@ -220,7 +220,9 @@ export default {
     </template>
 
     <template #cell(milestone)="{ item }">
-      <div :data-testid="$options.testIds.MILESTONE">{{ item.milestone.title }}</div>
+      <div v-if="item.milestone" :data-testid="$options.testIds.MILESTONE">
+        {{ item.milestone.title }}
+      </div>
     </template>
 
     <template #cell(pipelines)="{ item }">

ee/spec/frontend/analytics/merge_request_analytics/components/throughput_table_spec.js

@@ -199,8 +199,20 @@ describe('ThroughputTable', () => {
         expect(findCol(TEST_IDS.TIME_TO_MERGE).text()).toBe('4 minutes');
       });
 
-      it('displays the correct milestone', () => {
-        expect(findCol(TEST_IDS.MILESTONE).text()).toBe('v1.0');
+      it('does not display a milestone if not present', () => {
+        expect(findCol(TEST_IDS.MILESTONE).exists()).toBe(false);
+      });
+
+      it('displays the correct milestone when available', async () => {
+        const title = 'v1.0';
+
+        additionalData({
+          milestone: { title },
+        });
+
+        await wrapper.vm.$nextTick();
+
+        expect(findCol(TEST_IDS.MILESTONE).text()).toBe(title);
       });
 
       it('displays the correct pipeline count', () => {

ee/spec/frontend/analytics/merge_request_analytics/mock_data.js

@@ -63,7 +63,7 @@ export const throughputTableData = [
     createdAt: '2020-08-06T16:53:50Z',
     mergedAt: '2020-08-06T16:57:53Z',
     webUrl: 'http://127.0.0.1:3001/gitlab-org/gitlab-shell/-/merge_requests/11',
-    milestone: { title: 'v1.0' },
+    milestone: null,
     assignees: {
       nodes: [
         {