Check mAdnCache is not null before loading FDN. am: fd39f04d49

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/opt/telephony/+/24377542

Change-Id: I564089f119be5d96663d1a26cbd3880b885fd8a8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>