i have managed to work around this by creating a new collection 'foo0' with the same data. checking that i could see that on the dropdown then deleting that and recreating 'foo' now works fine. foo now appears...
i don't know enough about the internals to make any sensible guess about a cause here... its certainly not obvious like in the case where i had the empty texture and i am unfortunately no longer able to reproduce the problem.
i'm /guessing/ that some validation check of some kind failed, or the thing making the atlas failed in some unexpected way but silently (although i can certainly see an atlas texture)... if this is the case I would make the suggestion of failing loudly so the user is aware (e.g. like when sprites are too large to fit in an atlas)
EDIT: FYI my use of the empty texture was with dicing on and trimming off - my hope was that it would create a single small tile of nothing to build the larger sprite from so that i could use them as 'spacers' in some (admittedly fairly nasty) code we have which picks its sprites by index in their collection.
EDIT2: Although note - i still had the same problem with my animated sprite and animation clip/collection which referred to the sprite collection, remaking that fixed that as well.