Make sure that `attribute_filters` cannot be set to `{"key": []}`

[tcompa]

For the moment, such value would be accepted by the backend - but its meaningless because it would exclude all images. This may change, backend-side, and we may actually make it invalid.
Client-side, we should never prepare such a request body.

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@tcompa Agreed to all! Looks good to me. I'll review additional type filter questions on that issue separately

attribute_filters={"key1": []}

I have no strong opinion on this. In general, we should not allow people to actually set such filters in the interface.

Originally posted by @jluethi in #2182