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.