when resolver finds a zone cut from cache, it checks whether there is an empty non-terminal between target QNAME and cached zone cut.
this is indicated by presence of NODATA/NXDOMAIN in packet cache.
if it finds one, it turns off qname minimisation and continues,
this saves one query for empty non-term zones like ‘co.jp’

caveat: only direct child of the cut can be considered (e.g. ‘co.jp’ for ‘jp’), otherwise we would leak information to parent if the zone cut fell out of cache and NODATA existed