ActionCable: CreateMixin properties are all optional. (#39366)
ActionCable: CreateMixin properties are all optional. (#39366)

See:
- https://guides.rubyonrails.org/action_cable_overview.html#subscriber
- https://guides.rubyonrails.org/action_cable_overview.html#client-server-interactions-subscriptions
- https://github.com/rails/rails/blob/master/actioncable/app/javascript/action_cable/subscription.js#L59-L67
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