Future proof xml parsing in graphml. (networkx#7360)

As of Python 3.12, the lxml tree parser raises a warning about
evaluating truthiness of elements directly, instead recommending
an is not None or len() test.

networkx/readwrite/graphml.py

@@ -982,7 +982,7 @@ def decode_data_elements(self, graphml_keys, obj_xml):
                 node_label = None
                 # set GenericNode's configuration as shape type
                 gn = data_element.find(f"{{{self.NS_Y}}}GenericNode")
-                if gn:
+                if gn is not None:
                     data["shape_type"] = gn.get("configuration")
                 for node_type in ["GenericNode", "ShapeNode", "SVGNode", "ImageNode"]:
                     pref = f"{{{self.NS_Y}}}{node_type}/{{{self.NS_Y}}}"