Available harvesters can be identified as the empty string.

When running the NetarchiveSuite, the HarvestJobManager continuously logs its dispatch state:

Apr 27, 2011 4:21:38 PM dk.netarkivet.harvester.scheduler.HarvestDispatcher$DispatchState updateHarvesterStatus
INFO: Available harvesters:
LOWPRIORITY: [, low]
HIGHPRIORITY: [, high]

Apr 27, 2011 4:21:41 PM dk.netarkivet.harvester.scheduler.HarvestDispatcher$DispatchState updateHarvesterStatus
INFO: Available harvesters:
LOWPRIORITY: [low]
HIGHPRIORITY: [, high]

It seems, that available harvesters can be identified by the empty string!