Have a field x2many without relation_field. Before this commit, a server error was raised because, we send changes for the non-existing field (relation_field). Now, we only send to the server the changes for relation_field only if they exist. Note that, this commit will also add an exception on the server mock onchange if we sent changes for non-existings fields. Part of task~3179751 Part-of: odoo/odoo#114024