Allow sending of empty SMS

Description:

Sometimes it is required to send an empty SMS to, e.g., a service number (or similar). But this is currently not possible because the Send button is disabled as long as there is no text entered.

Improved behavior:

If the "Send" button is pressed and no text has yet been entered, a request should appear asking whether the user really wants to send an empty SMS. Choices "Cancel" and "Send anyway".

(Edit: Corrected bad english in "Improved behaviour" section)