fix(server): set default doc title to Untitled on email (#11152)

close CLOUD-181

packages/backend/server/src/__tests__/mails.spec.ts

@@ -1,6 +1,7 @@
 import test from 'ava';
 
 import { Renderers } from '../mails';
+import { TEST_DOC, TEST_USER } from '../mails/common';
 
 test('should render emails', async t => {
   for (const render of Object.values(Renderers)) {
@@ -9,3 +10,14 @@ test('should render emails', async t => {
     t.snapshot(content.html, content.subject);
   }
 });
+
+test('should render mention email with empty doc title', async t => {
+  const content = await Renderers.Mention({
+    user: TEST_USER,
+    doc: {
+      ...TEST_DOC,
+      title: '',
+    },
+  });
+  t.snapshot(content.html, content.subject);
+});