Fix engine crashing when using Down Arrow selection on Tree with no selection

[TheSofox]

Fixes #92958

Now when Down (or Up) key is used to change selection on a Tree with SELECT_MULTI enabled, the code will check whether anything is selected before trying to change the selection. Previously it would not perform this check and a crash would be possible if nothing was selected.

This has a different approach from #92959, though aiming to solve the same bug.

[KoBeWi]

When nothing is selected, you can't use arrows or any other button to select something. Which means that you can't select any item when you are Tabbing into the tree, making it less friendly for keyboard controls.

#92959 seems like a better approach, but it still has the problem I mentioned in my comment. Code of both PRs looks fine though.

[TheSofox]

Amended commit to be more like #92959 Seemed to fix the issues @Hilderin raised.
Since the issue was raised on the other PR, I tried to reproduce the "selecting folded nodes" glitch but it seemed to work okay. If anyone reproduces it, let me know with repo steps.

In general, this PR seems to work well.

[KoBeWi]

If anyone reproduces it, let me know with repo steps.

1. Have some folded node at the bottom of the scene.
2. Deselect all nodes.
3. Save the scene.
4. Reload the scene.
5. Click empty space in scene tree.
6. Press Up.

[TheSofox]

Thanks for the repo steps! The problem was get_last_item() didn't take into consideration whether the last item was hidden by a collapsed parent or not. Apparently this function is only used by _go_up(), so I modified it to only choose the last visible item.

[akien-mga]

Thanks!