Fix tracking on empty control point groups

[OliBomby]

Small issue I noticed. When adding a control point you might at some point have a control point group with no control points in it. This could happen because you for instance deselect the 'effects' section and then select the 'timing' section. However the moment you deselect the 'effects' section and there are no sections active, your selection gets moved to the previous effect control point group because of tracking. The control point group with no control points is very hard to select because the game remembers the tracked type and tries to select those types of control point groups.

This PR fixes the issue.

One unintended behaviour of this change is that when a control point group with no control points is selected, tracking is disabled, so it will not automatically move to the next control point group. I think this is fine though since the area of effect of this control point group with no control points is undefined so technically there is no "next group" which overrides the effect.

[peppy]

I guess this is okay for now because it was severely broken. But the eventual goal would have the "groups" hidden to the user (which may mean just removing the group when there's no types assigned to it automatically).