The icon logic in _ViewProviderWall.getIcon previously switched to the
assembly icon for any wall without a Base that contained Additions. This
caused an unexpected icon change when a wall was debased, as it still
retains a parametric body defined by its Length property.
Update the logic to only show the assembly icon if the wall has no Base,
has Additions, and its Length is zero. This ensures that baseless walls
driven by their own dimensions maintain the standard wall icon.
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