Add proper null check for task item rendering

If description is empty, this.$el was still non-null, but
it was just an HTML comment as a string. So the call to
querySelectorAll failed

app/assets/javascripts/issues/show/components/description.vue

@@ -95,7 +95,7 @@ export default {
     this.renderGFM();
     this.updateTaskStatusText();
 
-    if (this.workItemsEnabled && this.$el) {
+    if (this.workItemsEnabled) {
       this.renderTaskActions();
     }
   },
@@ -157,7 +157,12 @@ export default {
       }
     },
     renderTaskActions() {
+      if (!this.$el?.querySelectorAll) {
+        return;
+      }
+
       const taskListFields = this.$el.querySelectorAll('.task-list-item');
+
       taskListFields.forEach((item, index) => {
         const button = document.createElement('button');
         button.classList.add(

spec/frontend/issues/show/components/description_spec.js

@@ -209,6 +209,27 @@ describe('Description component', () => {
   });
 
   describe('with work items feature flag is enabled', () => {
+    describe('empty description', () => {
+      beforeEach(async () => {
+        createComponent({
+          props: {
+            descriptionHtml: '',
+          },
+          provide: {
+            glFeatures: {
+              workItems: true,
+            },
+          },
+        });
+        await nextTick();
+      });
+
+      it('renders without error', () => {
+        expect(findTaskActionButtons()).toHaveLength(0);
+      });
+    });
+
+    describe('description with checkboxes', () => {
       beforeEach(async () => {
         createComponent({
           props: {
@@ -262,4 +283,5 @@ describe('Description component', () => {
         expect(wrapper.text()).toContain(newTitle);
       });
     });
+  });
 });