WebGLApplication: handle single-node skeletons.
WebGLApplication: handle single-node skeletons.

Turns out the createArbor returns an Arbor with a null root when
the skeleotn has a single node, givne that no edges are created
and therefore the vertices array, which is a list of pairs of vertices,
 is empty.

fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID