gh-35511: Fix Graph.add_clique() for one vertex
Before the change:
sage: g = Graph()
sage: g.add_clique([1,2])
sage: g.add_clique([3])
sage: g.vertices(sort=True)
[1, 2]
After the change:
sage: g = Graph()
sage: g.add_clique([1,2])
sage: g.add_clique([3])
sage: g.vertices(sort=True)
[1, 2, 3]
### :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 created tests covering the changes.
URL: https://github.com/sagemath/sage/pull/35511
Reported by: Gonzalo Tornaría
Reviewer(s): David Coudert
