gh-35533: Fix bug in graph.maximum_average_degree
    
Fixes #35532.

### 📚 Description

Fix corner cases (empty graph, graph without edges) in
`graph.maximum_average_degree`.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. It should be `[x]` not `[x
]`. -->

- [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.
- [x] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: #35533
Reported by: David Coudert
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/sage/graphs/graph.py

@@ -4673,9 +4673,30 @@ def maximum_average_degree(self, value_only=True, solver=None, verbose=0):
             sage: mad_g = g.maximum_average_degree(value_only=False)
             sage: g.is_isomorphic(mad_g)
             True
+
+        TESTS:
+
+        Check corner cases::
+
+            sage: Graph().maximum_average_degree(value_only=True)
+            0
+            sage: Graph().maximum_average_degree(value_only=False)
+            Graph on 0 vertices
+            sage: Graph(1).maximum_average_degree(value_only=True)
+            0
+            sage: Graph(1).maximum_average_degree(value_only=False)
+            Graph on 1 vertex
+            sage: Graph(2).maximum_average_degree(value_only=True)
+            0
+            sage: Graph(2).maximum_average_degree(value_only=False)
+            Graph on 1 vertex
         """
         self._scream_if_not_simple()
         g = self
+        if not g:
+            return ZZ.zero() if value_only else g.parent()()
+        elif not g.size():
+            return ZZ.zero() if value_only else g.parent()([[next(g.vertex_iterator())], []])
         from sage.numerical.mip import MixedIntegerLinearProgram
 
         p = MixedIntegerLinearProgram(maximization=True, solver=solver)