### 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)

if (collapsed == p_collapsed) {
		return;
	}
	collapsed = p_collapsed;
	queue_sort();
}
void SplitContainer::set_dragger_visibility(DraggerVisibility p_visibility) {
	if (dragger_visibility == p_visibility) {
		return;
	}
	dragger_visibility = p_visibility;
	queue_sort();
}
SplitContainer::DraggerVisibility SplitContainer::get_dragger_visibility() const {
	return dragger_visibility;
}
bool SplitContainer::is_collapsed() const {
	return collapsed;
}
void SplitContainer::set_vertical(bool p_vertical) {
	ERR_FAIL_COND_MSG(is_fixed, "Can't change orientation of " + get_class() + ".");
	vertical = p_vertical;
	update_minimum_size();
	_resort();
}
bool SplitContainer::is_vertical() const {