match does not have associated data and DNS_RBTFIND_EMPTYDATA is not set.
Return DNS_R_PARTIALMATCH instead if there is a superdomain with
data, or ISC_R_NOTFOUND if no superdomain has data.

Make dns_rbt_findname() honor DNS_RBTFIND_EMPTYDATA, per the rbt.h
documentation that has been saying it will.  It didn't.