Spring Incorrect check in AbstractBrokerRegistration's constructor

The constructor in org.springframework.messaging.simp.config.AbstractBrokerRegistration checks the non-nullness of the clientOutboundChannel parameter twice but does not check the non-nullness of the clientInboundChannel parameter.

See: https://github.com/spring-projects/spring-framework/blob/959e6d17458dbef0bf3572b8c0fe33d714e7c2f0/spring-messaging/src/main/java/org/springframework/messaging/simp/config/AbstractBrokerRegistration.java#L48

Comment From: lower-case

Hi @jhoeller I've raised a PR to fix this extra assertion check. Please review and provide your feedback.

Thanks!