It seems that using an empty list when using the `__in=` filter prevents an
Aggregate Coalesce from working properly. See below:
# Test with matched Queryset. Sum will return 50
OrderItem.objects.filter(pk__in=[1]).aggregate(test=Coalesce(Sum('quantity'),
Value(0)))
>>> {'test': 50}
# Test with unmatched Queryset. Sum will return 0
OrderItem.objects.filter(pk__in=[-1]).aggregate(test=Coalesce(Sum('quantity'),
Value(0)))
>>> {'test':0}
# Test with unmatched Queryset (using empty list). Sum will return NONE
OrderItem.objects.filter(pk__in=[]).aggregate(test=Coalesce(Sum('quantity'),
Value(0)))
>>> {'test': None}
Can someone confirm? I will post as a bug if so.
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