Fix: KeyError: 'method' when build run_graphrag (#7899)

### What problem does this PR solve?
Close #7879
I checked the current master code, the kb_parser_config is join from
knowledge table, so I think should be some edge cases due to history
data

### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)

graphrag/general/index.py

@@ -57,7 +57,7 @@ async def run_graphrag(
 
     subgraph = await generate_subgraph(
         LightKGExt
-        if row["kb_parser_config"]["graphrag"]["method"] != "general"
+        if "method" not in row["kb_parser_config"]["graphrag"] or row["kb_parser_config"]["graphrag"]["method"] != "general"
         else GeneralKGExt,
         tenant_id,
         kb_id,