gh-36275: `sage.graphs.generic_graph`: some care with return ... else statements
    
We change this kind of statements:
```py
if ....:
    return ...
else:
    return ...
```
to
```py
if ....:
    return ...
return ...
```

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### ⌛ Dependencies

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URL: #36275
Reported by: David Coudert
Reviewer(s): David Coudert, Frédéric Chapoton

Author: Release Manager