Even lacking a motherboard and other normal components?
It depends on the psu... some have more internal loading
(power resistors) than others. Some might even shut off
froma rail being oo-spec too much. If that happened, you
could put a dummy load of some sort on the highest rail (%
over it's spec off the most), typically being the 5V rail
presuming the currently proposed load is fans on the 12V
rail.
Other psu, maybe most would run but have oltage off-spec
still. It's not a problem per se to do it then but why??
If you justwant fans you might consider a regular 12V approx
1A DC wall-wart. It will waste less power, a multi-hundred
watt ATX psu is not meant to have such a light load and
efficiency not tuned for it. It also puts wear on only a $5
psu (or many old consumer electronics could be cannibalized
for one instead, for free). That'd take a bit of soldering
though, if you wanted a reliable electrical joint.
