Use Option.ofNullable instead of Optional.of

Optional.of expects a non-null value, and throws a
NullPointerException if it gets null.

This needs to be fixed to upgrade errorprone to v3.8.1.

Bug: 190944875
Test: Presubmits
Change-Id: Ib7dd4df9a435a99adb46e69bc048016690f3d25a