Prevent Empty commands or messages from being sent

Summary:
Before:
You could press send on the command widget and AtCore would send an empty message of \n\r to the printer. This could cause the machine to stop responding as atcore is waiting on a new ready state if the machine does not send "ok" after the empty command
After:
if there is no text to send there is no emit.

Reviewers: laysrodrigues, tcanabrava, patrickelectric

Reviewed By: laysrodrigues, patrickelectric

Subscribers: Atelier: AtCore

Tags: Atelier: AtCore

Differential Revision: https://phabricator.kde.org/D17566