How does the 'Ferocity' Universal Monster Ability work for creatures of the Undead type? It seems like, since Undead lack a Constitution score, the creature would be staggered and lose additional hp over time yet never reach a negative hit point total equal to their constitution score (since they don't have one). Is this accurate? Does it matter that undead are "immediately destroyed when reduced to 0 hit points."?
Ferocity doesn't benefit undead creatures
Ferocity allows that a creature:
remains conscious and can continue fighting even if its hit point total is below 0.
But undead creatures can't reach negative hit points and fall unconscious; they are simply destroyed upon reaching 0hp. Ferocity prevents the creature from falling unconscious due to negative hit points, but it does not protect the creature from any of the other consequences of hit point loss, which for undead creatures means they are destroyed before ferocity is ever relevant.
But if you want it to, use their charisma score instead
If you houserule that ferocity does prevent the undead creature from being destroyed at 0hp, you should probably use their charisma score in place of their constitution score to determine how much more damage they can take. As per the description of Constitution and the Undead type:
Undead use their Charisma score in place of their Constitution score when calculating hit points, Fortitude saves, and any special ability that relies on Constitution (such as when calculating a breath weapon’s DC).
Undead explicitly base their hit point calculation and special abilities that depend on constitution on their charisma instead, so it makes sense to use their charisma score for this purpose too.