irmin-pack: fix and test `stable_hash = 0`

[craigfe]

The irmin-pack test suite is currently failing when passed a store with stable_hash = 0. We should work out why this is the case & fix it, then add this to our test suite.

Originally posted by @Ngoguey42 in https://github.com/mirage/irmin/issues/1738#issuecomment-1024106099