Allow zero length keys in isc_hashmap

Allow zero length keys in isc_hashmap

In case, we are trying to hash the empty key into the hashmap, the key
is going to have zero length.  This might happen in the unit test.
Allow this and add a unit test to ensure the empty zero-length key
doesn't hash to slot 0 as SipHash 2-4 (our hash function of choice) has
no problem with zero-length inputs.