gh-38301: graph: modular decomposition of a single vertex should be a…

… single tree node

    
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The modular decomposition tree computed by Sage for a graph with a
single vertex is wrong (tested in Sage from 9.8-10.4.rc0)

```py
sage: Graph(1).modular_decomposition()
(PRIME, [0])
```

Sage returns a tree with a `PRIME` root with a single child representing
the vertex `0`, but according to the recursive definition of a modular
decomposition tree, it should be a single node tree ([see Wikipedia for 
example](https://en.wikipedia.org/wiki/Modular_decomposition#:~:text=As%
20a%20base%20case%2C%20if,is%20a%20single%20tree%20node.))

This PR fixes this issue. It is an easy fix, as the case of single
vertex graphs was treated separately in the `modular_decomposition`
method of `Graph`.
Two doctests were modified to reflect this changes.

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preview.

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URL: #38301
Reported by: cyrilbouvier
Reviewer(s): cyrilbouvier, David Coudert

src/sage/graphs/graph.py

@@ -8159,9 +8159,9 @@ def modular_decomposition(self, algorithm=None, style='tuple'):
         Singleton Vertex::
 
             sage: Graph(1).modular_decomposition()
-            (PRIME, [0])
+            0
             sage: Graph(1).modular_decomposition(style='tree')
-            PRIME[0[]]
+            0[]
 
         Vertices may be arbitrary --- check that :issue:`24898` is fixed::
 
@@ -8209,8 +8209,7 @@ def modular_decomposition(self, algorithm=None, style='tuple'):
         if not self.order():
             D = None
         elif self.order() == 1:
-            D = create_prime_node()
-            D.children.append(create_normal_node(self.vertices(sort=False)[0]))
+            D = create_normal_node(next(self.vertex_iterator()))
         else:
             D = habib_maurer_algorithm(self)