refine mindmap (#1844)

### What problem does this PR solve?



### Type of change

- [x] Refactoring

graphrag/index.py

         keyset.add(k)
         arr.append({
             "id": k,
-            "children": be_children(v, keyset) if isinstance(v, dict) else []
+            "children": be_children(v, keyset)
         })
     return arr