This only happens when no sheet's dimension is specified.
When filling empty cells with empty strings, we push these new cells with the correct cell index but they are added at the end of the cells array (normal PHP behavior). This means that we were going from `{[0] => 'A', [2] => 'C'}` to `{[0] => 'A', [2] => 'C', [1] => ''}`. We therefore need to sort the array to get the values in the correct order ( `{[0] => 'A', [1] => '', [2] => 'C'}`).
