Phone: Fix NPE in PhoneUtils::toLogSafePhoneNumber

When PhoneGlobals.DEBUG_LEVEL is set to 2, there
will be NullPointerException when receiving an
incoming call from an unknown number.

Change-Id: I2a730b891420695f3a410164e611b66cf757edc8
Signed-off-by: Bin Li <libin@marvell.com>