### 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)

fit_child_in_rect(first, Rect2(Point2(0, 0), Size2(get_size().width, middle_sep)));
		int sofs = middle_sep + sep;
		fit_child_in_rect(second, Rect2(Point2(0, sofs), Size2(get_size().width, get_size().height - sofs)));
	} else {
		if (is_layout_rtl()) {
			middle_sep = get_size().width - middle_sep - sep;
			fit_child_in_rect(second, Rect2(Point2(0, 0), Size2(middle_sep, get_size().height)));
			int sofs = middle_sep + sep;
			fit_child_in_rect(first, Rect2(Point2(sofs, 0), Size2(get_size().width - sofs, get_size().height)));
		} else {
			fit_child_in_rect(first, Rect2(Point2(0, 0), Size2(middle_sep, get_size().height)));
			int sofs = middle_sep + sep;
			fit_child_in_rect(second, Rect2(Point2(sofs, 0), Size2(get_size().width - sofs, get_size().height)));
		}
	}
	
	if (dragger_visibility == DRAGGER_VISIBLE && !collapsed) {
		dragging_area_control->show();
		int dragger_ctrl_size = MAX(sep, theme_cache.minimum_grab_thickness);
		if (vertical) {
			dragging_area_control->set_rect(Rect2(Point2(0, middle_sep - (dragger_ctrl_size - sep) / 2), Size2(get_size().width, dragger_ctrl_size)));
		} else {
			dragging_area_control->set_rect(Rect2(Point2(middle_sep - (dragger_ctrl_size - sep) / 2, 0), Size2(dragger_ctrl_size, get_size().height)));
		}
		dragging_area_control->queue_redraw();
	} else {
		dragging_area_control->hide();
	}
}
Size2 SplitContainer::get_minimum_size() const {