Wait for the empty content element to be shown

When the "Comments" tab is open the empty content element is always in
the DOM, although it is only shown once the message collection was
fetched and there were no messages. Due to this it is necessary to
explicitly wait for it to be shown instead of relying on the implicit
wait made to find the element; otherwise it would be found immediately
and if the collection was not fetched yet it would not be visible,
causing the test to fail.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>