Fall back to MXID when no display name is present

MemberAvatar requires a display name, or else it refuses to render.

Signed-off-by: Robin Townsend <robin@robin.town>

src/components/views/elements/EventTilePreview.tsx

@@ -101,7 +101,7 @@ export default class EventTilePreview extends React.Component<IProps, IState> {
 
         // Fake it more
         event.sender = {
-            name: this.props.displayName,
+            name: this.props.displayName || this.props.userId,
             userId: this.props.userId,
             getAvatarUrl: (..._) => {
                 return Avatar.avatarUrlForUser(