The bounds of a plot of the art's bounds no longer count the objects a
plot leaves out (placed images, objects at 0% opacity, unpainted paths)
when they sit in the art of a symbol. An instance whose symbol holds
nothing else is left out, so a document with only such instances is
refused with "nothing to export". The art of a dynamic symbol, and of a
group with effects, fills or strokes of its own, is measured whole.