chore: fix type to check whether it is None

Signed-off-by: makinzm <nozomi.maki.da@gmail.com>

Author: makinzm

sdk/python/feast/infra/online_stores/milvus_online_store/milvus.py

@@ -764,6 +764,7 @@ def _drop_all_version_collections(self, project: str, table: FeatureView) -> Non
         """
         base = f"{project}_{table.name}"
         versioned_prefix = f"{base}_v"
+        assert self.client is not None, "Milvus client is not initialized"
         for collection_name in self.client.list_collections():
             if collection_name == base or (
                 collection_name.startswith(versioned_prefix)