fix: correct [AttributeError: 'set' object has no attribute 'nodes' T… (#6699)

### Related Issue: 
https://github.com/infiniflow/ragflow/issues/6653 

### Environment:
Using nightly version [ece5903]

Elasticsearch database

Thanks for the review! My fault! I realize my initial testing wasn't
passed.

In graphrag/entity_resolution.py 
 `sub_connect_graph` is a set like` {'HELLO', 'Hi', 'How are you'}`, 
Neither accessing `.nodes` nor `.nodes()` will work, **it still causes
`AttributeError: 'set' object has no attribute 'nodes'`**

In graphrag/general/extractor.py  
The `list.extend() `method performs an in-place operation, directly
modifying the original list and returning ‘None’ rather than the
modified list.
Neither accessing
`sorted(set(node0_attrs[attr].extend(node1_attrs.get(attr, []))))` nor
`sorted(set(node0_attrs[attr].extend(node1_attrs[attr])))` will work,
**it still causes `TypeError: 'NoneType' object is not iterable`**
### Type of change

- [ ] Bug Fix AttributeError: graphrag/entity_resolution.py 
- [ ] Bug Fix TypeError: graphrag/general/extractor.py

graphrag/entity_resolution.py

         connect_graph.add_edges_from(resolution_result)
         async with trio.open_nursery() as nursery:
             for sub_connect_graph in nx.connected_components(connect_graph):
-                merging_nodes = list(sub_connect_graph.nodes())
+                merging_nodes = list(sub_connect_graph)
                 nursery.start_soon(lambda: self._merge_graph_nodes(graph, merging_nodes, change))
 
         # Update pagerank

graphrag/general/extractor.py

             # Merge two nodes, keep "entity_name", "entity_type", "page_rank" unchanged.
             node1_attrs = graph.nodes[node1]
             node0_attrs["description"] += f"{GRAPH_FIELD_SEP}{node1_attrs['description']}"
-            node0_attrs["source_id"] = sorted(set(node0_attrs["source_id"].extend(node1_attrs.get("source_id", []))))
+            node0_attrs["source_id"] = sorted(set(node0_attrs["source_id"] + node1_attrs["source_id"]))
             for neighbor in graph.neighbors(node1):
                 change.removed_edges.add(get_from_to(node1, neighbor))
                 if neighbor not in nodes_set:
                         edge0_attrs["weight"] += edge1_attrs["weight"]
                         edge0_attrs["description"] += f"{GRAPH_FIELD_SEP}{edge1_attrs['description']}"
                         for attr in ["keywords", "source_id"]:
-                            edge0_attrs[attr] = sorted(set(edge0_attrs[attr].extend(edge1_attrs.get(attr, []))))
+                            edge0_attrs[attr] = sorted(set(edge0_attrs[attr] + edge1_attrs[attr]))
                         edge0_attrs["description"] = await self._handle_entity_relation_summary(f"({nodes[0]}, {neighbor})", edge0_attrs["description"])
                         graph.add_edge(nodes[0], neighbor, **edge0_attrs)
                     else: