handle missing edge types in graph data

lightrag/types.py

@@ -1,5 +1,5 @@
 from pydantic import BaseModel
-from typing import List, Dict, Any
+from typing import List, Dict, Any, Optional
 
 
 class GPTKeywordExtractionFormat(BaseModel):
@@ -15,7 +15,7 @@ class KnowledgeGraphNode(BaseModel):
 
 class KnowledgeGraphEdge(BaseModel):
     id: str
-    type: str
+    type: Optional[str]
     source: str  # id of source node
     target: str  # id of target node
     properties: Dict[str, Any]  # anything else goes here