Skip creating a module if no function is going to be imported.
Also includes a change so that if the first partition is empty (which
can happen),
we import global with non-local linkage into the first non-empty
partition, instead
of P0 all the time.
I thought we'd need to change users of the SplitModule callback so they
can deal with less modules
than the number requested, but no. We already return only 1 module in
some cases and
it seems to be handled just fine.
Fixes SWDEV-523146
2168455ef4