Dialer: At least show "Bluetooth"

* Currently, if we can't get the bluetooth device alias, we get an
  empty string, which we also display, resulting in an empty line
  being shown
* Replace the empty string with the default "Bluetooth" string so
  we at least have something to show (and it's also accurate enough)

Change-Id: I2a260e12175fae2573c11980aa09e3483b679ef4