Laravel 3 Eloquent: Looking for a "where_nested()" example

Question

I have a custom validation rule that is checking the DB for NULL.
I need to look for null or empty.

HAVE:

    $query = $this->db()->table($table);
    ...
    foreach( $null_columns as $col )
            $query->where_null($col);

Which results in something like: SELECT * FROM t WHERE col1=foo AND col2 IS NULL

WANT:
SELECT * FROM t WHERE col1=blah AND (col2 = '' OR col2 IS NULL)

QUESTION:

Is where-nested() the right tool for the job?
If so, I'd really like to see an example.
If not, what's a good way to approach this?

Answer 1

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 :)