Make query_string argument to QueryDict optional

[duncan@…]

Previously, a blank QueryDict could be created by calling

QueryDict(str('')) or QueryDict(None)

It seems rather nicer to just allow the query_string argument to
default to None so that we can just write QueryDict().

It's a commit in ​​https://github.com/django/django/pull/2778

[Tim Graham <timograham@…>]

In d68987ae25f0e0fb30727473803758af53d75671:

Fixed #22897 -- Made QueryDict query_string argument optional.

Now QueryDict() is equivalent to QueryDict() or QueryDict(None).

[Tim Graham <timograham@…>]

In ead21a1:

Refs #22897 -- Removed unneeded empty string QueryDict argument.