SSH: Use "none" authentication type in case of an empty password.

The "password" type as used until now should also be okay, but does not
work with dropbear.

Task-number: QTCREATORBUG-14913
Change-Id: I2fac255acf326e509f68c9de97c3cc9f00b2a373
Reviewed-by: hjk <hjk@theqtcompany.com>