Undead are not immune to figments (such as mirror image)
Undead are immune to most mind-affecting spells (except those that specifically affect undead):
Immunity to all mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms).
Depite those effects in parenthesis being merely examples and not an exhaustive list, many illusions are not mind-affecting, as each subtype of illusion will state wether they are mind-affecting, quasi-real or something else. As such, unless the Illusion is a Pattern or a Phantasm, undead are not automatically immune to them.
Pattern: Like a figment, a pattern spell creates an image that others can see, but a pattern also affects the minds of those who see it or are caught in it. All patterns are mind-affecting spells.
Phantasm: A phantasm spell creates a mental image that usually only the caster and the subject (or subjects) of the spell can perceive. This impression is totally in the minds of the subjects. It is a personalized mental impression, all in their heads and not a fake picture or something that they actually see. Third parties viewing or studying the scene don't notice the phantasm. All phantasms are mind-affecting spells.
Spells like Illusory Wall, or Silent Image, work fine on non-intellingent undead, as they are Figments:
Figment: A figment spell creates a false sensation. Those who perceive the figment perceive the same thing, not their own slightly different versions of the figment. It is not a personalized mental impression. Figments cannot make something seem to be something else. A figment that includes audible effects cannot duplicate intelligible speech unless the spell description specifically says it can. If intelligible speech is possible, it must be in a language you can speak. If you try to duplicate a language you cannot speak, the figment produces gibberish. Likewise, you cannot make a visual copy of something unless you know what it looks like (or copy another sense exactly unless you have experienced it).
Because figments and glamers are unreal, they cannot produce real effects the way that other types of illusions can. Figments and glamers cannot cause damage to objects or creatures, support weight, provide nutrition, or provide protection from the elements. Consequently, these spells are useful for confounding foes, but useless for attacking them directly.
Which is also the reason why undead will not automatically detect someone affected by the Invisbility spell (a glamer). As such, undead will be affected by your illusory floor just like every other creature in the game.
They have to interact with the illusion before making checks
As noted under Saving Throws and Illusions (Disbelief), creatures have to somehow interact with an illusion before they are allowed a saving throw. This includes walking on the illusion, spending a move action to search for hidden traps as noted under Perception, throwing stones ahead of them and notice that some fell into the ground, or screaming that they fell into a trap despite the normal-looking floor (which grants a +4 on the check).
Creatures encountering an illusion usually do not receive saving throws to recognize it as illusory until they study it carefully or interact with it in some fashion.
A successful saving throw against an illusion reveals it to be false, but a figment or phantasm remains as a translucent outline.
A failed saving throw indicates that a character fails to notice something is amiss. a character faced with proof that an illusion isn’t real needs no saving throw. If any viewer successfully disbelieves an illusion and communicates this fact to others, each such viewer gains a saving throw with a +4 bonus.
Mindless undead's reactions are GM fiat
As for how they will react, that is entirely dependant on your GM and the orders that those mindless undead recieved from their master. Zombies and skeletons have no Int score, but are allowed Will saves normally. They are allowed a Will save to notice your illusion, but they might not be able to react on the pit trap based on the orders they recieved (and how your GM wants to roleplay those orders).
There are no hard rules on how the GM should play mindless creatures such as those, they may say they react on instinct, such as not pursuiting into a trap if that would hamper their orders, trying to find a way around the trap, or even trying to leap over the trap.
Personally, I would probably say that all the undead fall into the trap, then try to climb their way out if they survive it, as they have no distinction of "survivability" or sense of "pain" unless coming from a positive-energy source.
