Added support for emptyText option

- When no value is given, `selectedIndex` becomes -1 and `emptyText` is
rendered
- When value related properties is set and gives a non-exist value,
`selectedIndex` is set to -1 and `emptyText` is rendered
- When `datasource` is changed and does not previous value,
`selectedIndex` is set to -1 and `emptyText` is rendered