No Issue : Internal Refactoring to improve code readability.

[chickenchickenlove]

Motivation:

• There are some codes using *.size() > 0 or *.size() == 0 in spring-kafka.
• There is code where the constructor could be reused, but it was not.
• There is code where a null value can occur, but it does not indicate that null is possible.

Modifications:

• Replace a.size() > 0 with !a.isEmpty().
• Replace a.size() == 0 with a.isEmpty().
• Reuse constructor already existed to remove duplicate constructor logic.
• Add @Nullable annotations to field, and method. Also, add warning log.

Result:

• There is only one side effect which developer can observe.
  • when BatchMessagingMessageListenerAdapter#setBatchMessageConverter() is executed, new warning log can be occur.