[MEP] Handle java.lang.NullPointerException

The objects in the getUiccSlotsInfo return array can be null if the slot info is not available. It is possible in two cases
1. Slots info is no proper due to modem is in bad state
2. API is called between phone process starting and getting slot info from modem.

Bug: 227795919
Test: Manually tested esim scenarios on C10 device (Download, Enable, Disable & Delete profile) and atest FrameworksTelephonyTests
Change-Id: I031d0b2de6b98053d3f01a2284d43dd41081bfce