Fix date_select :selected option so you can pass it nil

I realised my previous change did not allow you to override the date to `nil`, since `nil || value`, will not return `nil`.

Updated to check if the `:selected` option is defined at all.

Guessing this doesn't need further changelog entry, as it's a follow up to previous change...?

References:
https://github.com/rails/rails/pull/8938
https://github.com/rails/rails/pull/8925