Forbid `attribute_filters={"key1": []}`

[tcompa]

Let's forbid attribute_filters={"key1": []}

See also:

• Make sure that attribute_filters cannot be set to {"key": []} fractal-web#700

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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