Remove TODO comment and unnecessary null checks for getBeanName()

[david-parkk]

Summary

This PR attempts to resolve the TODO comment in AbstractMessageListenerContainer.getBeanName() by adding @NonNull annotation and removing what appear to be unnecessary null checks in related classes.

Analysis

Based on my understanding of the codebase:

• The beanName field is declared as @NonNull with default value "noBeanNameSet"
• Therefore, getBeanName() should never return null
• The original @Nullable annotation seems to have been incorrect
• The null checks (getBeanName() == null ? "" : getBeanName()) appear to be unnecessary

Please let me know if I've misunderstood any aspect of this.

Changes

• AbstractMessageListenerContainer.java: Remove TODO comment, add @NonNull to getBeanName()
• KafkaMessageListenerContainer.java: Remove unnecessary null check
• ShareKafkaMessageListenerContainer.java: Remove unnecessary null check

Test

Added shouldSupportDefaultBeanNameSetting() test to verify that getBeanName() returns the default value "noBeanNameSet" instead of null when setBeanName() is not called.

---

I'm happy to make any adjustments based on your feedback. Thank you to the maintainers for taking the time to review this contribution!

[david-parkk]

I removed the @NonNull from the method since the field already has @NonNull annotation - adding it to the getter seemed redundant. Happy to add it back if you prefer explicit annotation on the method.

[artembilan]

Thank you!
Will be merged after successful build.