plugin XEP-0470: fix autocreate in `create_attachments_node`: Due to config check, `create_attachments_node` was failing when the target node was not already existing. However, this is the case when an item is published and `autocreate` is expected, thus the node is now created if not already existing and new `autocreate` argument is set.

[D-BUS Service]
Name=org.libervia.Libervia
Exec=libervia-backend