check if_null statement don't work

I simply want to check, if a query give me 'NULL' as the result.

    $check = '  SELECT some_data FROM user
                WHERE condition = my_condition LIMIT 1';
                if ( is_null($check) )
    {        
    echo "yep, it's null";
    }

But i don't get the echo. The query definitly gives me the result 'NULL'. Whats wrong?