Describe the issue
When using an empty POJO (class without fields), JsonMessageConverter converts it to null instead of a representation of an empty JSON ({}).

To Reproduce
Steps to reproduce the behavior:

1. Create POJO class e.g.

   public class EmptyPOJO {
   }

2. Use JsonMessageConverter#convertToInternal method to convert the POJO. It uses jsonMapper.toJson(payload) inside.
3. Returned value is null

Version of the framework
Spring Cloud Stream 4.0.4

Expected behavior
JSON message converter returns the representation of an empty JSON: {}

Additional context
Issue appeared when migrating from Spring Boot 2 to 3.0.12. Stream tests that we had passing in v2 started to fail, and we debugged the problem down to this issue.