Timestamp: Eliminate 'Invalid date' from potential output

ts/components/ConversationListItem.md

@@ -184,3 +184,28 @@ We only show one line.
   />
 </div>
 ```
+
+#### Missing data
+
+```jsx
+<div>
+  <ConversationListItem
+    name="John"
+    lastUpdated={null}
+    lastMessage={{
+      text: 'Missing last updated',
+    }}
+    onClick={() => console.log('onClick')}
+    i18n={util.i18n}
+  />
+  <ConversationListItem
+    name="Missing message"
+    lastUpdated={Date.now() - 5 * 60 * 1000}
+    lastMessage={{
+      text: null,
+    }}
+    onClick={() => console.log('onClick')}
+    i18n={util.i18n}
+  />
+</div>
+```

ts/components/conversation/Timestamp.tsx

@@ -7,7 +7,7 @@ import { formatRelativeTime } from '../../util/formatRelativeTime';
 import { Localizer } from '../../types/Util';
 
 interface Props {
-  timestamp: number;
+  timestamp: number | null;
   extended: boolean;
   module?: string;
   withImageNoCaption?: boolean;
@@ -52,6 +52,10 @@ export class Timestamp extends React.Component<Props> {
     } = this.props;
     const moduleName = module || 'module-timestamp';
 
+    if (timestamp === null || timestamp === undefined) {
+      return null;
+    }
+
     return (
       <span
         className={classNames(