Fallback against incorrect NXDOMAIN during iterative lookups

[khibino]

According to RFC7816 https://datatracker.ietf.org/doc/html/rfc7816#section-3
There are name server implementations that incorrectly return NXDOMAIN for ENTs (Empty Non-Terminals),
Full resolvers may receive NXDOMAINs during iterative lookups by performing QNAME minimization.
As a countermeasure against such cases, it is necessary to implement a fallback process like knot and bind9 do.
https://bind9.readthedocs.io/en/v9.18.14/reference.html#namedconf-statement-qname-minimization