[UICC] Fix NPE when reading port index without a SIM.

A NullPointerException would occur when trying to read the port index
from a UiccSlot when no SIM card was present.
This change adds the necessary null checks to prevent the crash.

Bug: 436942206
Test: tests/telephonytests/src/com/android/internal/telephony/uicc/UiccControllerTest.java
Flag: EXEMPT bugfix
Change-Id: I8a412a18f1878280737128a87d30fc87098f4b0c