core (xmpp): Prevent non printable message to be displayed:

We now use `client.is_message_printable` to check if message is printable. If it's not, we
block storage in DB and sending to bridge. The previous code could let message pass if
metadata were stored in `extra`.

libervia/backend/core/xmpp.py

@@ -1840,7 +1840,7 @@ class SatMessageProtocol(xmppim.MessageProtocol):
             await post_treat
 
             self.complete_attachments(data)
-            if not data["message"] and not data["extra"] and not data["subject"]:
+            if not client.is_message_printable(data):
                 raise exceptions.CancelError("Cancelled empty message")
             if not client.is_component or client.receiveHistory:
                 await self.add_to_history(data)