menu.mediaType can be an enum value, which includes 0 to represent a
web page link, or null if no link was right clicked.
Check for null explicitely when evaluating whether the menu entry
should be displayed.
If user right clicked on the most recent event in the timeline
(index 0), a new message was posted in the room (the new index 0),
and the user right clicked on it, the EventContextMenu's `event`
property relying on the index would not detect any change and
thus would still target the previous event.
This affected options depending on this property like "Reply".
The problem was reproducible with any same index repetition, not just 0.
