Fix: Handle the case of deleting empty blocks. Update the relevant message (#6643)

…gic to return the correct deletion message. Add handling for empty
arrays to ensure no errors occur during the deletion operation. Update
the test cases to verify the new logic.

### What problem does this PR solve?

fix this bug：https://github.com/infiniflow/ragflow/issues/6607

### Type of change

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

---------

Co-authored-by: wenju.li <wenju.li@deepctr.cn>

sdk/python/test/test_http_api/test_chunk_management_within_dataset/test_delete_chunks.py

@@ -168,7 +168,8 @@ class TestChunksDeletion:
             ),
             (lambda r: {"chunk_ids": r[:1]}, 0, "", 4),
             (lambda r: {"chunk_ids": r}, 0, "", 1),
-            pytest.param({"chunk_ids": []}, 0, "", 0, marks=pytest.mark.skip(reason="issues/6607")),
+            pytest.param({"chunk_ids": []}, 0, "deleted 5 chunks", 0)
+
         ],
     )
     def test_basic_scenarios(