Returns an empty {@code Nullable} instance. No value is present for this
{@code Nullable}.
@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 Nullable.empty()}. There is no guarantee that it is a singleton.
Instead, use {@link #isPresent()}.
@param U The type of the non-existent value
@return an empty {@code Nullable}
Returns an empty {@code Nullable} instance. No value is present for this {@code Nullable}.
@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 Nullable.empty()}. There is no guarantee that it is a singleton. Instead, use {@link #isPresent()}.
@param U The type of the non-existent value @return an empty {@code Nullable}