Fix cagra::extend error message (#532)

When extending a CAGRA index that is built with `index_param.attach_dataset_on_build = false`, an error message "Only uncompressed dataset is supported" is displayed even if the dataset used to build the graph is not compressed. This problem occurs since the extend function does not check whether the dataset is empty. This PR fixes it.

Authors:
  - tsuki (https://github.com/enp1s0)

Approvers:
  - Artem M. Chirkin (https://github.com/achirkin)

URL: #532

Author: enp1s0

cpp/src/neighbors/detail/cagra/add_nodes.cuh

@@ -432,8 +432,14 @@ void extend_core(
     } else {
       index.update_graph(handle, raft::make_const_mdspan(updated_graph.view()));
     }
+  } else if (dynamic_cast<const cuvs::neighbors::empty_dataset<int64_t>*>(&index.data()) !=
+             nullptr) {
+    RAFT_FAIL(
+      "cagra::extend only supports an index to which the dataset is attached. Please check if the "
+      "index was built with index_param.attach_dataset_on_build = true, or if a dataset was "
+      "attached after the build.");
   } else {
-    RAFT_FAIL("Only uncompressed dataset is supported");
+    RAFT_FAIL("cagra::extend only supports an uncompressed dataset index");
   }
 }
 }  // namespace cuvs::neighbors::cagra