We have a family iMac that is used for little more than browsing the internet and printing things off. There is only one user with no password. Previously this was no issue, we could either tap enter or click the arrow and we would be logged in. As of one of the more recent operating system updates this no longer works however (perhaps even since Mojave, we don't use this Mac very often). Tapping enter does nothing and the arrow only shows when you enter characters into the field. Fortunately the guest account is enabled which works for now but it would be good to understand how to log into an account when you have no password as it won't let us in!
P.S. I know the account has no password (and no password works) because when I am in the guest account and navigate to settings I can do certain activities using that admin login. Unfortunately this does not include changing the password of the family account.