gh-35533: Fix bug in graph.maximum_average_degree
Fixes #35532.
### :books: Description
Fix corner cases (empty graph, graph without edges) in
`graph.maximum_average_degree`.
### :memo: Checklist
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/35533
Reported by: David Coudert
Reviewer(s): Travis Scrimshaw
