don't show allocated tasks if there are none
don't show allocated tasks if there are none

This is done to be consistent with periodic runs, which
are also not shown if there are no periodic runs. If we now
add allocation information also to periodic runs, it would be
confusing otherwise. Also the UI looks less clumsy like this.

FRosner/cluster-broccoli,FRosner/cluster-broccoli,FRosner/cluster-broccoli