Fix (sdk): ensure variables defined in rm_chunk API (#12274)

### What problem does this PR solve?

Fixes a bug in the `rm_chunk` SDK interface where an `UnboundLocalError`
could
occur if `chunk_ids` is not provided in the request. 

- `unique_chunk_ids` and `duplicate_messages` are now always initialized
  in the `else` branch when `chunk_ids` is missing.
- API behavior remains unchanged when `chunk_ids` is present.

### Type of change

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

api/apps/sdk/doc.py

@@ -1286,6 +1286,9 @@ async def rm_chunk(tenant_id, dataset_id, document_id):
     if "chunk_ids" in req:
         unique_chunk_ids, duplicate_messages = check_duplicate_ids(req["chunk_ids"], "chunk")
         condition["id"] = unique_chunk_ids
+    else:
+        unique_chunk_ids = []
+        duplicate_messages = []
     chunk_number = settings.docStoreConn.delete(condition, search.index_name(tenant_id), dataset_id)
     if chunk_number != 0:
         DocumentService.decrement_chunk_num(document_id, dataset_id, 1, chunk_number, 0)