Fixed crash in getPhoneNumber

getPhoneNumber used to return empty string when
passed in subscription id is invalid. Adapted the
behavior to avoid clients crash.

Fix: 268535693
Test: *#*#4646#*#* without SIM inserted
Test: atest SubscriptionManagerServiceTest
Change-Id: Ifae48a1d358892797dd51b84baf923d1b4321dba