misc(treemap): fix node coverage shading

[connorjclark]

We added this in #12486 but turns out the logic was inverted.

ref #11254

[brendankenny]

with 8.0 out and turning our attention to polishing, is there some room for adding tests for this kind of thing? It's hard to catch coverage problems visually from debug.json (e.g. I assume this fixes bullet #3 in #11254 (comment) but the problem wasn't obvious) but would be relatively easy to test programmatically (e.g. 10% unused bytes and getComputedStyle(el, '::before').width is 10% of el's width)