Java: When calling getContent or getItems, get null if the other property was set.

[matthewbolanos]

In Java, when creating a ChatMessageContent, you can either set the content or items of the object. If you then retrieve either, the system isn't able to smartly pull from the right source.

For example, if I set content to "hello world" and I call getItems(), I should receive an array of KernelContent with a single TextContent inside of it. The reverse should also be true.