gh-40927: fix issue 40885: restore correct setting of attribute `_embedding`
    
Fixes #40885.

We restore the correct setting of attribute `_embedding` after the call
to `is_planar` in method `faces` of graphs.
The attribute is modified only when the graph is planar.


### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [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 and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - #12345: short description why this is a dependency -->
<!-- - #34567: ... -->
    
URL: #40927
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton

Author: Release Manager

src/sage/graphs/generic_graph.py

@@ -6947,6 +6947,15 @@ def faces(self, embedding=None):
 
             sage: graphs.PathGraph(3).faces()
             [[(0, 1), (1, 2), (2, 1), (1, 0)]]
+
+        Check that :issue:`40885` is fixed::
+
+            sage: g = graphs.CycleGraph(3)
+            sage: hasattr(g, '_embedding')
+            False
+            sage: F = g.faces()
+            sage: hasattr(g, '_embedding')
+            False
         """
         if not self.order() or not self.size():
             return []
@@ -6957,9 +6966,13 @@ def faces(self, embedding=None):
         else:
             embedding = self.get_embedding()
             if embedding is None:
+                has_attribute = hasattr(self, '_embedding')
                 if self.is_planar(set_embedding=True):
                     embedding = self._embedding
-                    self._embedding = None
+                    if has_attribute:
+                        self._embedding = None
+                    else:
+                        del self._embedding
                 else:
                     raise ValueError("no embedding is provided and the graph is not planar")