Fix node merge error

lightrag/operate.py

@@ -256,7 +256,7 @@ async def _merge_nodes_then_upsert(
     already_file_paths = []
 
     already_node = await knowledge_graph_inst.get_node(entity_name)
-    if already_node is not None:
+    if already_node:
         already_entity_types.append(already_node["entity_type"])
         already_source_ids.extend(
             split_string_by_multi_markers(already_node["source_id"], [GRAPH_FIELD_SEP])