WL#2267: Make HA_NULL_IN_KEY part of index_flags (instead of table_flags)

HA_NULL_IN_KEY is currently a table flag forcing a handler
to support NULL searches on all indexes or none. In ndbcluster
pure UNQIUE INDEXES (USING HASH) do not support NULL search.
Allowing to declare this in the index_flags instead would signal to
the optimizer not to do NULL searches for these kind of indexes.