code: public static <T> StreamEx<T> ofNullable(T element) {
          return element == null ? empty() : of(element);
      }
docs: Returns a sequential {@code StreamEx} containing a single element, if
  non-null, otherwise returns an empty {@code StreamEx}.
  
  @param element the single element
  @param <T> the type of stream elements
  @return a stream with a single element if the specified element is
  non-null, otherwise an empty stream
  @since 0.1.1