Fix AST If/Else branch drawing
Fix AST If/Else branch drawing

An orphan branch was still drawn after the If body, even when
no else was present. `last` is now bound to the absence of else
branch.

amyinorbit/orbitvm,cesarparent/orbitvm,amyinorbit/orbitvm,amyinorbit/orbitvm,cesarparent/orbitvm