Now QNetworkCookie::parseCookies just ignores empty domain attribute instead of ignoring entire cookie.

But rfc-6265 defines different:
http://www.rfc-base.org/txt/rfc-6265.txt:

 If the attribute-value is empty, the behavior is undefined.  However,
   the user agent SHOULD ignore the cookie-av entirely.

https://bugreports.qt.io/browse/QTBUG-15794 - https://codereview.qt-project.org/#/c/28396/

Just an example: https://developer.gnome.org/libsoup/stable/SoupCookie.html#soup-cookie-parse
If domain is NULL this is not a valid state for a SoupCookie, and you will need to fill in some appropriate string for the domain if you want to actually make use of the cookie.