make column#limit and column#null work properly

column#limit was broken, always returning nil due to the fact that ActiveRecord::ConnectionAdapters::Column requires a sql type to be passed to the constructor while nulldb passes a logical type. Alternative to current fix would be to redefine the limit method on each instance of column to return a static limit.

column#null needs to return true/false whereas it used to return nil for columns where a value wasn't set explicitly.