The rules don’t indicate anything other than the spell simply being negated on a successful save, so I suppose that’s what happens. The creature actually moving (changing squares) and the spell coming into effect but being empty would require special rules; by default, Reflex negates means that a succssful Reflex save eliminates the spell entirely.
This seems really dumb to me, but that’s what it is.
The spell is always centered around a creature and its description indicates that
A globe of shimmering force encloses a creature
Apparently you cannot enclose more than one creature, even if the size is large enough. How that works, I have no idea, since yes, the sphere can get large enough that it seems like it ought to be able to do so.
The wording on this spell is really bizarre. It seems to be split between two ideas, not sure which one it wants to be: either the spell is specifically an enclosure for a single creature (which must be small enough to fit in a CL-ft. radius sphere), or else the spell creates a CL-ft. radius sphere, inside which a creature (or creatures) could be trapped.
By my reading, the spell seems to mostly be the first; there’s no indication that you can create an empty resilient sphere, and there’s the fact that a Reflex save is supposed to negate the spell entirely. However, the lack of something like it being an “up to CL-ft. radius sphere” or something would seem to imply that you always get the full size, even if that’s way bigger than the trapped creature.
The alternative seems contrary to how the spell is worded (i.e. they are very specific about the sphere trapping a creature consistently throughout the spell). If the effect of the spell was simply to create a sphere, which may or may not have a creature in it, I would have expected very different phrasing, emphasizing the sphere rather than the creature. Instead, there would just be rules for what happens if any creatures happen to be in the sphere (and you would expect a Reflex save to let them dive out of the sphere, not negate it). The way the spell is worded doesn’t really support this. Which is unfortunate, because at least to me it seems much more sensible.