Make `Q(Q(), ...)` equivalent to `Q(...)`

[jonathan-golorry]

Empty Q objects (Q() or ~Q(), etc) evaluate to False, but Q(Q()) evaluates to True. This interferes with the shortcut on logical operations involving empty Q objects and can lead to bugs when trying to detect empty operations.

def q_any(iterable):
    q = Q()
    for element in iterable:
        q |= element
    if q:
        return q
    return Q(pk__in=[])
Item.objects.filter(q_any())  # no items
Item.objects.filter(q_any([Q()]))  # no items
Item.objects.filter(q_any([Q(Q())]))  # all items

This patch removes empty Q objects from args during Q object initialization.

This requires https://code.djangoproject.com/ticket/32548 in order to prevent a regression in logical operations between query expressions and empty Q objects.