Enable using an empty order_by() to disable implicit primary key ordering in first()

[Lily]

As discussed in ​https://github.com/django/new-features/issues/71, update the interaction between order_by and first to enable this behaviour:

# Existing
MyModel.objects.first()  # first, ordered by `pk`
MyModel.objects.order_by("pk").first()  # as above
MyModel.objects.order_by("name").first()  # first, ordered by `name`

# New
MyModel.objects.order_by().first()  # first, but in whatever order the database returns it

This will interact with ​default orderings (`Meta.ordering`), which will need some careful handling.