Remove paginator's unordered queryset warning for a .none() queryset

[Jeremy Lainé]

Paginator shows a warning if it receives an unordered queryset.

However, if also emits this warning if passed a queryset which is guaranteed to be empty, for instance Article.objects.none().

Working around this warning involves doing a bizarre Article.objects.none().order_by('id')

I believe there can be legitimate usecases for passing a .none() queryset to a paginator, for instance if a user's permissions are such that we restrict the objects they can view to the empty queryset.

[Junji Wei]

PR ​https://github.com/django/django/pull/9479

[Carlton Gibson]

Discussion on PR led to conclusion that this could be resolved by having QuerySet.ordered return True when the queryset is empty. (Something is ordered if it always returns the same sequence. An empty QuerySet does, i.e. [])

Marking as "Patch needs improvement" pending that change.

[Carlton Gibson]

​New PR from Josh Schneier.

Minor change to docstring suggested but otherwise, looks good.

[Tim Graham <timograham@…>]

In 4d48ddd:

Fixed #28917 -- Prevented Paginator's unordered warning on EmptyQuerySet.

Thanks carltongibson for the idea and weijunji for the initial patch.