Code should honor empty lists/sets/etc and not treat it as None

[daviddavis]

In the constants file, there is an empty constant that lumps None and other empty values together. Several places use this constant to ignore values when they are None/Empty (e.g. here).

The problem is that when you call /users?id__in= for example, you'd think that no users would be returned. The query is effectively asking for users whose ids are in [] (which is no users). I'd argue that asking for users with id__in=[] is very different than id__in=None since the latter means that no filter was supplied/no filtering is desired. To give an example using django:

In [3]: User.objects.all()
Out[3]: <QuerySet [<User: admin>]>

In [4]: User.objects.filter(pk__in=[])
Out[4]: <QuerySet []>