gh-67041: Allow to distinguish between empty and not defined URI components

Changes in the urllib.parse module:

* Add option allow_none in urlparse(), urlsplit() and urldefrag(). If
  it is true, represent not defined components as None instead of an
  empty string.
* Add option keep_empty in urlunparse() and urlunsplit(). If it is
  true, keep empty non-None components in the resulting string.
* Add option keep_empty in the geturl() method of DefragResult,
  SplitResult, ParseResult and the corresponding bytes counterparts.

Author: serhiy-storchaka