Added null check while fetching the context URI resource

This CL aims at adding a null check before we convert the fetched URI
character sequence to string format.

Bug: b/257012915
Test: Tested by building and flashing on local, and seeing everything
works okay as expected before for 3P livewallpapers.

Change-Id: Id90367ee16d479591e524d7c2c5774ec3c483628