gh-36634: permutation_normal_form : case of empty matrices
fixes #36629
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
URL: https://github.com/sagemath/sage/pull/36634
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
