public static <T> Optional<T> empty() {
        @SuppressWarnings("unchecked")
        Optional<T> t = (Optional<T>) EMPTY;
        return t;
    }
Returns an empty {@code Optional} instance.  No value is present for this
Optional.

@param <T> Type of the non-existent value
@return an empty {@code Optional}
@apiNote Though it may be tempting to do so, avoid testing if an object
is empty by comparing with {@code ==} against instances returned by
{@code Option.empty()}. There is no guarantee that it is a singleton.
Instead, use {@link #isPresent()}.
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/util/Optional.java