initialized carrier_certificate_string_array to an empty array

instead of null, so that when a certificate is removed, it is also
removed in SubscriptionInfoUpdater#updateSubscriptionByCarrierConfig

Bug: 162700101
Test: manual
Change-Id: I921007498f42119581f565fcc6625c1275c59df8
Merged-In: I921007498f42119581f565fcc6625c1275c59df8