Merge "Nullability check for mount module." am: 7b90c81b7c

Original change: https://android-review.googlesource.com/c/platform/bionic/+/2539232

Change-Id: I8e50a657521d875581224b6d7fcddb1b84bc5ec6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>