### Godot version

4.0.dev (cd031fd31a8fdcdae1e93d1b20caf93230f0ac59)

### System information

Windows 10

### Issue description

When using the generic SplitContainer (not the H/V class variations), with the default theme, the grabber icon is missing, both when the vertical bool is set to true or false:

![image](https://user-images.githubusercontent.com/5117197/213923999-2762f456-5bc1-4541-afa2-f39de051b4ad.png)


### Steps to reproduce

- Create SplitContainer (not HSplitContainer or VSplitContainer)
- Give it two children so it renders properly
- Hover grabber in-game

### Minimal reproduction project

[split_container_icon.zip](https://github.com/godotengine/godot/files/10474735/split_container_icon.zip)

switch (p_what) {
		case NOTIFICATION_MOUSE_ENTER: {
			mouse_inside = true;
			SplitContainer *sc = Object::cast_to<SplitContainer>(get_parent());
			if (sc->get_theme_constant(SNAME("autohide"))) {
				queue_redraw();
			}
		} break;
		case NOTIFICATION_MOUSE_EXIT: {
			mouse_inside = false;
			SplitContainer *sc = Object::cast_to<SplitContainer>(get_parent());
			if (sc->get_theme_constant(SNAME("autohide"))) {
				queue_redraw();
			}
		} break;
		case NOTIFICATION_DRAW: {
			SplitContainer *sc = Object::cast_to<SplitContainer>(get_parent());
			if (!dragging && !mouse_inside && sc->get_theme_constant(SNAME("autohide"))) {
				return;
			}
			Ref<Texture2D> tex = sc->get_theme_icon(SNAME("grabber"));
			draw_texture(tex, (get_size() - tex->get_size()) / 2);
		} break;
	}
}
Control *SplitContainer::_getch(int p_idx) const {