code: public function nullsFirst() : Ordering
      {
          return Ordering::from(Collections::comparatorFrom(
              function ($object1, $object2) {
                  return $object1 === null
                      ? -1
                      : ($object2 === null
                          ? 1
                          : $this->compare($object1, $object2));
              }
          ));
      }
docs: Returns an ordering that treats null as less than all other values and uses this to compare non-null values.
  
  @return Ordering