Would it make sense to make the column non-nullable and always set it to infinity when there is no expiry?
A password is not required for roles. In many cases, external authentication is used in ph_hba.conf. I think it would be strange to have 'infinity' for roles without a password.
Tom suggested to have 'infinity' in the \du output for roles with a password. My doubt is that this will hide the real values (absence of values). So I suggested a separate column 'Has password?' to show roles with password and unmodified column 'Password expire time'.
Yes, it's easy to replace NULL with "infinity" for roles with a password, but why? What is the reason for this action? Absence of value for 'expire time' clear indicates that there is no time limit. Also I don't see a practical reasons to execute next command, since it do nothing:
ALTER ROLE .. PASSWORD 'infinity';
So I think that in most cases there is no "infinity" in the rolvaliduntil column.