### 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)

int idx = 0;
	for (int i = 0; i < get_child_count(false); i++) {
		Control *c = Object::cast_to<Control>(get_child(i, false));
		if (!c || !c->is_visible()) {
			continue;
		}
		if (c->is_set_as_top_level()) {
			continue;
		}
		if (idx == p_idx) {
			return c;
		}
		idx++;
	}
	return nullptr;
}
Ref<Texture2D> SplitContainer::_get_grabber_icon() const {
	if (is_fixed) {
		return theme_cache.grabber_icon;
	} else {
		if (vertical) {
			return theme_cache.grabber_icon_v;
		} else {
			return theme_cache.grabber_icon_h;
		}
	}
}
void SplitContainer::_compute_middle_sep(bool p_clamp) {