CI_DB_result::row() is supposed to return an stdClass object encapsulating a single row from the query result. When there are no rows in the result, row() returns an empty array instead, which is very unexpected for a method that's supposed to return an object — expected behavior would be to return null.

I've tracked this to line 255 in system/database/DB_result.php, in the row_object() method:

{{{

!php

if (count($result) == 0)
{
return $result;
}
}}}

Edit: this apparently applies to all row-fetching methods as well (first_row(), last_row(), next_row(), previous_row()).