Well, I ended up getting it to work this way:
$query = $this->db()->table($table);
...
foreach( $null_columns as $col )
{
$query->where(function($q) use($col){
$q->where($col,'=','');
$q->or_where_null($col);
});
}
... but would still like to see an example of using where_nested()
to do this, if possible. I always appreciate learning something new :)