Set initial value and handle validation with unselectedText properly

I'm not completely sure about this check, as it'll also apply to any select options that intentionally set value to an empty string eg.
```
new SelectView({
    name: 'option',
    options: [ ['signup', 'Signup'], ['', 'Reset'] ]
})
```
This looks like edgecase to me though.
One solution could be forcing usage of non-empty values and verifying it in `getOptionValue` function.
However this might sound not so user-friendly.