code: @SafeVarargs
      public final StreamEx<T> ifEmpty(T... values) {
          return ifEmpty(null, Spliterators.spliterator(values, Spliterator.ORDERED));
      }
docs: Returns a stream which contents is the same as this stream, except the case when
  this stream is empty. In this case its contents is replaced with supplied values.
  
  <p>
  This is a <a href="package-summary.html#StreamOps">quasi-intermediate
  operation</a>.
  
  @param values values to replace the contents of this stream if this stream is empty.
  @return the stream which contents is replaced by supplied values only if this stream is empty.
  @since 0.6.6