Incorrect check in AbstractBrokerRegistration's constructor

[tgyurci]

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:

spring-framework/spring-messaging/src/main/java/org/springframework/messaging/simp/config/AbstractBrokerRegistration.java

Line 48 in 959e6d1

Assert.notNull(clientOutboundChannel, "'clientInboundChannel' must not be null");

[lower-case]

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

Thanks!