plugin pubsub cache: replace empty list by None in getItemsFromCache:

if an empty list is used, no item can be retrieved, this patch replace empty list by None
to avoid this case.

sat/plugins/plugin_pubsub_cache.py

@@ -508,7 +508,7 @@ class PubsubCache:
         if max_items is None and rsm_request is None:
             max_items = 20
             pubsub_items, metadata = await self.host.memory.storage.getItems(
-                node, max_items=max_items, item_ids=item_ids,
+                node, max_items=max_items, item_ids=item_ids or None,
                 order_by=extra.get(C.KEY_ORDER_BY)
             )
         elif max_items is not None: