fix empty mind map issue (#1991)

### What problem does this PR solve?

### Type of change

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

graphrag/mind_map_extractor.py

@@ -105,6 +105,9 @@ class MindMapExtractor:
             for i, _ in enumerate(threads):
                 res.append(_.result())
 
+            if not res:
+                return MindMapResult(output={"root":{}})
+
             merge_json = reduce(self._merge, res)
             if len(merge_json.keys()) > 1:
                 keyset = set(