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: I38b5d80ec8c1e2dc93d76e08a0d0051fc4127ab3