gh-35700: Fix corner case of ordered set partitions iteration
### :books: Description
The function `multiset_permutation_next_lex` was wrong on empty input
and is fixed in this PR.
Fixes #35654
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/35700
Reported by: Vincent Delecroix
Reviewer(s): Frédéric Chapoton
