Empty and nested optgroup elements

Click on the select element above. When it drops down you should see the following items in the list:

- Item one
- Group one
- Group two
- (unselectable, empty row)
- Item inside an optgroup without a label
- (unselectable, empty row)
- Item inside an optgroup with an empty label
- (selectable, empty row)
- Group three
- Item two
- Nested group 1
- Nested group 2
- Nested group 3
- Item three
- Item four

The text of the rows of the dropdown must not overlap each other.