plugin XEP-0473: Don't try do load secrets if there is not encrypted elements:

The plugin was trying to decrypt all pubsub events, without checking if there is
"encrypted" element first, resulting in warning for all clear events.

This patch fixes it by detecting <encrypted> element and exiting early if none is found.

libervia/backend/plugins/plugin_xep_0473.py

@@ -757,6 +757,14 @@ class XEP_0473:
                 f"Invalid event's items element, missing node: {items_elt.toXml()}"
             )
             return
+
+        has_encrypted = any(
+            next(item_elt.elements(NS_OXPS, "encrypted"), None)
+            for item_elt in items_elt.elements(pubsub.NS_PUBSUB_EVENT, "item")
+        )
+        if not has_encrypted:
+            return
+
         node_uri = uri.build_xmpp_uri("pubsub", path=pubsub_service.full(), node=node)
         shared_secrets = await self.load_secrets(client, node_uri)
         if shared_secrets is None: