admin: Return QuerySet when a filter has no input
If a filter has not been given an input, then django-filter should normally not call its function. However, it seems this is not the case with the index Volume filter, whose function is called with no value and as a result, it returns an empty QuerySet. Fix this issue by checking in filter_index if it has been passed with any parameters. Also, since this issue may appear in other filter functions too, add the same check in every function that doesn't have it. NOTE: Model filters have a decorator that automatically does this check for them.