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

[serhiy-storchaka]

Changes in the urllib.parse module:

• Add option ~allow_none~ missing_as_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. By default it is the same as the allow_none value for the result of the urlparse() and urlsplit() calls.
• ~Add option keep_empty in the geturl() method of DefragResult, SplitResult, ParseResult and the corresponding bytes counterparts.~

• Issue: gh-67041