MessageReceiver and Sender Security Issue

[MoeweX]

The sign and verify methods of the message class depend on the content field. However, in some cases the receiver does not have a value set for the content field, so he does not sign the message, which is accepted by the sender.

This needs to be changed in that the content field is never null, so sign and verify work again. The tolerating code needs to be removed from the sender and receiver as well (null check for content before sign/verify).