Sending a message with a single whitespace throws an error

[nsarrazin]

If you try to send a message with a single whitespace the UI will let you do it and it will result in an error. This is because we trim the message in the backend but do a simple length > 0 check in the front-end.

We should probably not trim user messages in the backend, since if users want to experiment with some tokenization related things we should probably not manipulate the user input in any way.