Shuffle hangs if there is an empty shard

[andyleiserson]

See this CI failure: https://github.com/private-attribution/ipa/actions/runs/12934964616/job/36077201921?pr=1524 (or output.txt, in case the CI run expires).

There is only one input record, which means that one of the shards is empty.

The proptest seed is:

proptest: Saving this and future failures in /home/runner/work/ipa/ipa/ipa-core/proptest-regressions/protocol/hybrid/breakdown_reveal.txt
proptest: If this test was run on a CI system, you may wish to add the following line to your copy of the file. (You may need to create it.)
cc 6f36e60c1ca2a62199852ddc3cfe573945677da908386169cded086e56e54844

The TestWorld seed is 7357173389116502538 (but that should be derived from the proptest seed)

I recently increased the timeout for this test from 30 to 60s, prompted by a previous failure. That looks like it was actually the same issue, so the timeout increase was probably not necessary.