Validate required fields and fail gracefully if they're not present

[derek63]

Instead of waiting for NullPointerExceptions to occur, like when you don't specify a payload.

[alechenninger]

I think we should actually just not require a payload for the case where we are sending a message to EMA precisely because it does not have a payload. It may still have headers which can be used to track its origin, therefore it's still useful to have messages that have no payloads.