In project with only one clip, focus gets stuck

### Description

Currently, if a clip has focus and you press down arrow key the focus will move to the next clip below. However, this causes an issue whereby if the User has only one clip but multiple tracks and tries to move the track focus down to the next track, it won't move as their is no clip for it to jump to. Please remove this behaviour and instead make it so that if the User presses down arrow key the track focus shifts to the next track and is focused.

No file or test is identified. Reproduce the issue in an Audacity project with one clip and multiple tracks, then locate the clip and track focus handling for the down-arrow action. Done means pressing down moves focus to the next track even when there is no next clip.

Written by the indexing model from the issue text.