add support for nil schema
add support for nil schema

keeguon/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,keeguon/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,keeguon/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter