Wizard Alice casts Antimagic Field on herself around the corner from Paladin Bob. Bob, not realising this, then walks round the corner and casts Zone of Truth on an area containing Alice.
Obviously the effects of the Zone of Truth are suspended around Alice, so she does not make any saving throws against the spell.
However, Zone of Truth has the following clause:
You know whether each creature succeeds or fails on its saving throw.
What does Bob know about Alice's success or failure of the saving throw she never made? Does he "know" that she "succeeded"? Or does he know nothing, and therefore know something has gone wrong with the spell by his conspicuous lack of knowledge about Alice's save?