Changed null checks in Passpoint APIs to return non-null values

These Passpoint APIs are designated @NonNull, and so their return
values should be empty collections if a null message is received
from the AsyncChannel.

Bug: 129485601
Test: atest ClientModeImplTest
Change-Id: I1d9e864c67581fa8a4ad593b3e73b6ea6c7eb15b
2 files changed