[1.5.x] Avoided a possible regression in 5097d3c5.

QuerySet.none() returns an instance of EmptyQuerySet, which may have
undesirable side effects in the presence of custom query set classes.

The implementation of .none() was refactored in master to have the same
effect as .filter(pk__in=[]).

Refs #19652.

Thanks Simon Charrette for the report.