core (main): fix bridge return value of message_send:

`message_send` was returning message data while the method is declared
with no return value, this was causing issues with some bridges (pb
notably).

libervia/backend/core/main.py

@@ -1099,9 +1099,12 @@ class LiberviaBackend(service.Service):
     ):
         client = self.get_client(profile_key)
         to_jid = jid.JID(to_jid_s)
-        return client.sendMessage(
+        d = client.sendMessage(
             to_jid, message, subject, mess_type, data_format.deserialise(extra_s)
         )
+        # sendMessage returns message data, but we must return nothing.
+        d.addCallback(lambda __: None)
+        return d
 
     def _set_presence(self, to="", show="", statuses=None, profile_key=C.PROF_KEY_NONE):
         return self.presence_set(jid.JID(to) if to else None, show, statuses, profile_key)