### 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)

} else {
		wished_middle_sep = split_offset_with_collapse;
	}
	
	middle_sep = CLAMP(wished_middle_sep, ms_first, size - sep - ms_second);
	
	if (p_clamp) {
		split_offset -= wished_middle_sep - middle_sep;
	}
}
void SplitContainer::_resort() {
	Control *first = _getch(0);
	Control *second = _getch(1);
	
	if (!first || !second) {
		if (first) {
			fit_child_in_rect(first, Rect2(Point2(), get_size()));
		} else if (second) {
			fit_child_in_rect(second, Rect2(Point2(), get_size()));
		}
		dragging_area_control->hide();
		return;
	}
	
	_compute_middle_sep(false);
	
	Ref<Texture2D> g = _get_grabber_icon();
	int sep = (dragger_visibility != DRAGGER_HIDDEN_COLLAPSED) ? MAX(theme_cache.separation, vertical ? g->get_height() : g->get_width()) : 0;
	
	if (vertical) {