fix mx_Event containment rules and empty read avatar row

res/css/structures/_RoomView.scss

@@ -222,7 +222,6 @@ limitations under the License.
 
 .mx_RoomView_MessageList li {
     clear: both;
-    contain: content;
 }
 
 li.mx_RoomView_myReadMarker_container {

src/components/views/rooms/EventTile.tsx

@@ -631,7 +631,7 @@ export default class EventTile extends React.Component<IProps, IState> {
 
         // return early if there are no read receipts
         if (!this.props.readReceipts || this.props.readReceipts.length === 0) {
-            return (<span className="mx_EventTile_readAvatars" />);
+            return null;
         }
 
         const ReadReceiptMarker = sdk.getComponent('rooms.ReadReceiptMarker');