The `.empty` map in a shard is weird: it claims to have capacity 1, but
you're not actually allowed to actually use that capacity. That's fine
for the normal insertion algorithm, because it always resizes to a
higher capacity when inserting the initial element. However,
`rehashTrackedInsts` was not aware of this caveat, so sometimes tried to
store to the single element of the `empty` map.

This system exists to avoid an extra branch in the main resizing logic
(since `new_cap = old_cap * 2` only works if the capacity is never
non-zero). However, it's fine for `rehashTrackedInsts` to have an extra
branch to handle this case, since it's literally called once per update.