### 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_popup_menu = style_popup->duplicate();
	
	
	
	style_popup_menu->set_content_margin_individual(EDSCALE, 2 * EDSCALE, EDSCALE, 2 * EDSCALE);
	
	style_popup_menu->set_border_width_all(EDSCALE);
	if (draw_extra_borders) {
		style_popup_menu->set_border_color(extra_border_color_2);
	} else {
		style_popup_menu->set_border_color(dark_color_2);
	}
	theme->set_stylebox("panel", "PopupMenu", style_popup_menu);
	Ref<StyleBoxFlat> style_menu_hover = style_widget_hover->duplicate();
	
	style_menu_hover->set_corner_radius_all(0);
	theme->set_stylebox("hover", "PopupMenu", style_menu_hover);
	theme->set_stylebox("separator", "PopupMenu", style_popup_separator);
	theme->set_stylebox("labeled_separator_left", "PopupMenu", style_popup_labeled_separator_left);
	theme->set_stylebox("labeled_separator_right", "PopupMenu", style_popup_labeled_separator_right);
	theme->set_color("font_color", "PopupMenu", font_color);
	theme->set_color("font_hover_color", "PopupMenu", font_hover_color);
	theme->set_color("font_accelerator_color", "PopupMenu", font_disabled_color);
	theme->set_color("font_disabled_color", "PopupMenu", font_disabled_color);
	theme->set_color("font_separator_color", "PopupMenu", font_disabled_color);
	theme->set_icon("checked", "PopupMenu", theme->get_icon(SNAME("GuiChecked"), SNAME("EditorIcons")));
	theme->set_icon("unchecked", "PopupMenu", theme->get_icon(SNAME("GuiUnchecked"), SNAME("EditorIcons")));
	theme->set_icon("radio_checked", "PopupMenu", theme->get_icon(SNAME("GuiRadioChecked"), SNAME("EditorIcons")));
	theme->set_icon("radio_unchecked", "PopupMenu", theme->get_icon(SNAME("GuiRadioUnchecked"), SNAME("EditorIcons")));