Empty non-terminals are not handled correctly yet

Consider the following zone for test.:

$ORIGIN test.
; omitting SOA and NS records for test.
c.b.a.nested.test.  IN TXT "Found"

Assuming no other resource record exists in the zone, queries for b.a.nested.test, a.nested.test and nested.test should return NOERROR with no data.

Currently, however, IBDNS returns NXDOMAIN for these cases.

This is the result of a naive implementation that could be interesting to keep. As always, the correct behavior should be implemented and made to be the default, but a configuration key should make it possible to revert to the erroneous implementation.