### 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)

}
	Ref<StyleBoxFlat> style_widget_disabled = style_widget->duplicate();
	if (draw_extra_borders) {
		style_widget_disabled->set_border_color(extra_border_color_2);
	} else {
		style_widget_disabled->set_border_color(disabled_color);
	}
	style_widget_disabled->set_bg_color(disabled_bg_color);
	Ref<StyleBoxFlat> style_widget_focus = style_widget->duplicate();
	style_widget_focus->set_draw_center(false);
	style_widget_focus->set_border_width_all(Math::round(2 * MAX(1, EDSCALE)));
	style_widget_focus->set_border_color(accent_color);
	Ref<StyleBoxFlat> style_widget_pressed = style_widget->duplicate();
	style_widget_pressed->set_bg_color(dark_color_1.darkened(0.125));
	Ref<StyleBoxFlat> style_widget_hover = style_widget->duplicate();
	style_widget_hover->set_bg_color(mono_color * Color(1, 1, 1, 0.11));
	if (draw_extra_borders) {
		style_widget_hover->set_border_color(extra_border_color_1);
	} else {
		style_widget_hover->set_border_color(mono_color * Color(1, 1, 1, 0.05));
	}
	
	Ref<StyleBoxFlat> style_popup = style_default->duplicate();
	const int popup_margin_size = default_margin_size * EDSCALE * 3;
	style_popup->set_content_margin_all(popup_margin_size);
	style_popup->set_border_color(contrast_color_1);
	const Color shadow_color = Color(0, 0, 0, dark_theme ? 0.3 : 0.1);
	style_popup->set_shadow_color(shadow_color);
	style_popup->set_shadow_size(4 * EDSCALE);