I don't ever remember seeing that accent in use in Norway?!
It can be omitted, so you can write both 'en' and 'én'. But when you're using the accent, you're distinguishing between 'one' and 'a/an' more easily.
It's just like in Dutch, when you put an accent on the letter 'e'; when the Dutch word 'een' becomes 'één' like the Norwegian language 'en' to 'én'.
Is the accent optional in Dutch?
Either way, een means both one in Dutch. So more specifically; they are interchangeable.
We mostly use two diacritic signs (acute accent and circumflex), mostly in loanwords, person-names and geographical names. But you don't need to include them.
Why is this in the "absence" skill? Did you really mean zero?