Allow empty `ConsistentPermutation`

Is there any reason `ConsistentPermutation` doesn't allow `n` to be zero - as per [this assertion](https://github.com/github/rust-gems/blob/main/crates/consistent-choose-k/src/consistent_permutation.rs#L156).

This results in special casing when you're constructing the permutations. We might need a branch at the top of the `next` function to check, but it should be highly predictable.

Start in crates/consistent-choose-k/src/consistent_permutation.rs, especially the assertion at line 156 and the next function mentioned in the issue. Verify how n = 0 is currently handled and confirm that empty permutations can be constructed without caller-side special casing.

Written by the indexing model from the issue text.