@SuppressWarnings("unchecked")
    @Override
    public <A> A[] toArray(IntFunction<A[]> generator) {
        checkConsumed();
        requireNonNull(generator);
        final A[] result = generator.apply(1);
        result[0] = (A) element;
        return result;
    }
Creates and returns an optimized sequential {@link Stream} implementation
with only a single element if the element is non-null. if the element is
null, an empty Stream is returned.

@param element the single element or null
@param <T> the type of the stream elements
@return a singleton sequential stream
common-parent/singletonstream/src/main/java/com/speedment/common/singletonstream/SingletonStream.java