Fix: handle sometimes graph index will miss explanation (#7127)

### What problem does this PR solve?

https://github.com/infiniflow/ragflow/issues/7053

### Type of change

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

graphrag/general/index.py

@@ -277,7 +277,7 @@ async def extract_community(
     for stru, rep in zip(community_structure, community_reports):
         obj = {
             "report": rep,
-            "evidences": "\n".join([f["explanation"] for f in stru["findings"]]),
+            "evidences": "\n".join([f.get("explanation", "") for f in stru["findings"]]),
         }
         chunk = {
             "id": get_uuid(),