Fix Nullability annotation

The initial commit of the @Nullable annotation appears
to have been based off of the javadocs. However, the actual
code does not appear to have allowed for null at that time.

It's not entirely clear if null was ever allowed, but it
hasn't been allowed since API 14 at least

Test: make
Fixes: 195924626
Change-Id: Ibffdc7b90cbfc3bc65a4a08bee52e2e3412bed3e