This is similar to UUM-128598 which was discussed "as designed" at the time, but after more discussions we have a new plan.

From [~martinpa] :

• All the lists we have should support {{{}none{}}}.
• We don't have a concept of an empty list in {{{}UXML{}}}.
• When we have an {{UXML}} inline value set for a list, removing the last item should set it to {{{}none{}}}.
• Removing the {{UXML}} inline value should go through the menu item.
• We should add support for {{Set Value}} through the menu item so an empty list can be set.
• We should fix any bugs related to having an empty list in style resolution.

The reason I think this is that unless we support {{none}} for empty lists, there is no way to actually override something coming from a selector that has set it. You can set it to a different "value", but you can't opt-out of it.
 
Important:
The filter style field currently implements the wrong behavior, and should be adjusted as part of fixing this issue:
[https://github.cds.internal.unity3d.com/unity/unity/pull/93390/files#r896153]