Remove an unnecessary ContextMenu Separator

When right clicking on a simple image there would be a separator on the
top of the DefaultContextMenu. This patch fixes it by checking if the
context menu has any elements in it before adding a separator.

Change-Id: I6bbe91fdac9f49006be8cc73df1d97d34f3c2848
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>