This improves performance of rendering for large trees.

They don't provide useful information and for large trees, they are a
big performance hit.

When a child doesn't have children, avoid API first-tree calls.

Merge remote-tracking branch 'origin/668-phylo-on-the-exported-svg-file-fix-the-background-color-behind-the-label' into dev

(the flake build does take long, and it's only a dev dependency anyways)