Add null check when loading telephony subscriptions

SubscriptionManager.getActiveSubscriptionInfoLis() can return null and this
wasn't being handled. The linked bug was already fixed by a different change
but the additional check is good to do.

Test:
manual verify that ImportDialogFragment works with and without a SIM card

Bug 32556709

Change-Id: I325cb30b5559626d91f0f79aaa1c99b98a39b070