Dialer: Fix possible NPE

* In some cases with VVM involved, the phone number seems to be null,
  resulting in a NPE when trying to split it by the regex
* Make sure there is always a valid string involved to prevent the error

BUGBASH-1447, BUGBASH-1242

Change-Id: I88302deeaeb3e085190fdab5fa7d2a8030bceb88