Rename NullHstore value to Hstore
Rename NullHstore value to Hstore

Ensure consistency with other Null* types

silviucm/pgx,jackc/pgx,kelvich/pgx,deoxxa/pgx,silviucm/pgx,plopik/pgx,rsto/pgx,jonasi/pgx,jackc/pgx,plopik/pgx,plopik/pgx,jackc/pgx,jackc/pgx,josephglanville/pgx,silviucm/pgx