Invalid AMQP message with missing application-data section

The AMQP spec writes clearly and unambiguously that each AMQP message must have at least once application-data (body) section:

The body section may denote an empty body though.
However, it's not allowed to completely omit the body section.
This library however sends AMQP messages completely omitting the body section, which is a bug.

Reproduction:
```fsharp
let message = new Message(Properties = new Properties())
```

More context in https://github.com/rabbitmq/rabbitmq-server/pull/15048 and https://github.com/rabbitmq/rabbitmq-server/pull/14967

Start with the reproduced `new Message(Properties = new Properties())` case and trace how that message is serialized. Confirm that an empty body still emits an application-data section, then verify the resulting message is accepted as valid AMQP.

Written by the indexing model from the issue text.