Settings: only set phone number if it's not empty

Only set the phone number in Rename mobile network dialog if it's not
empty in order to preserve the ´Not available´ -text.

Test: m, open ´SIMs´ in settings, click Pen-icon to edit SIM, verify
      either mobile number or ´Not available´ -text is shown.
Change-Id: I3bc1f1f4eb26cda38e5cc7a12bd4db1bf922f076