There's not much point in giving nonliving creatures nausea immunity, because AFAICT there's no nauseating effect that can touch them anyway. The vast majority allow Fortitude saves; a few inflict nausea by mental compulsion. Either way, they don't work on constructs or undead. Arguing whether they have literal nausea immunity is like arguing the distinction between infinite AC and infinite DR-- though they're conceptualized very differently, it's an academic discussion because the eventual game effect is identical.
IMO the current situation, without a specific nausea immunity, is slightly preferable because it has greater flexibility. Imagine a hypothetical new cleric spell that applies the nauseated condition to an undead, by divine power, without being mind-affecting or allowing a Fort save. If undead had nausea immunity, the description of this new spell would have to duplicate the effects of the nauseated condition-- the inability to cast spells, do anything requiring attention, and so on-- without actually using the word "nauseated." But as the rules are written, it would work just fine.