REST doesn't allow setting header_filter action to None
Valid actions (actually jump chain names) for a header_filter rule are hold, reject, discard, accept and None with None meaning use the configured [antispam]jump_chain: value
as the action, but the REST HeaderMatch class validates the action with enum_validator(Action)
which accepts only hold, reject, discard, accept and defer as valid actions, thus precluding setting None.