Allow zero length keys in isc_hashmap
In case, we are trying to hash the "root zone" into the hashmap, the key is going to have zero length. Allow this and add a unit test to ensure this doesn't hash to slot 0.
In case, we are trying to hash the "root zone" into the hashmap, the key is going to have zero length. Allow this and add a unit test to ensure this doesn't hash to slot 0.