Interactive card details form

I focused my self in the mobile version of the challenge. My questions are about the form validation part: 1)how do I check if the input value is empty? with ' input.value == "" ' or ' input.value == null '? there are some differences? 2)how do I 'force' the space between the numbers in the card number input? I mean both while writing and when I show it in the interactive card.

Thanks for help, any other tip is welcome.