#9 Allow for empty repeater-type OIDs

[Wim Nelis]

Some repeater-type OIDs contain a varying number of elements and might even be empty. An example is the list of associated clients on an access-point. Monitoring those OIDs should not result in an error condition (Missing repeater data for primary OID) if there are zero elements in the list.
An optional parameter in the oids file for a 'branch' OID to specify that an empty list might occur for this OID is probably needed.