### 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)

style_tab_selected->set_bg_color(base_color);
	
	style_tab_selected->set_border_width(SIDE_TOP, Math::round(2 * EDSCALE));
	
	Color tab_highlight = dark_color_2.lerp(accent_color, 0.75);
	style_tab_selected->set_border_color(tab_highlight);
	style_tab_selected->set_corner_radius_all(0);
	Ref<StyleBoxFlat> style_tab_unselected = style_tab_base->duplicate();
	style_tab_unselected->set_expand_margin(SIDE_BOTTOM, 0);
	style_tab_unselected->set_bg_color(dark_color_1);
	
	style_tab_unselected->set_border_color(Color(0, 0, 0, 0));
	Ref<StyleBoxFlat> style_tab_disabled = style_tab_base->duplicate();
	style_tab_disabled->set_expand_margin(SIDE_BOTTOM, 0);
	style_tab_disabled->set_bg_color(disabled_bg_color);
	style_tab_disabled->set_border_color(disabled_bg_color);
	
	Color background_color_opaque = background_color;
	background_color_opaque.a = 1.0;
	theme->set_color("background", "Editor", background_color_opaque);
	theme->set_stylebox("Background", "EditorStyles", make_flat_stylebox(background_color_opaque, default_margin_size, default_margin_size, default_margin_size, default_margin_size));
	
	theme->set_stylebox("Focus", "EditorStyles", style_widget_focus);
	
	Ref<StyleBoxFlat> style_widget_focus_viewport = style_widget_focus->duplicate();
	style_widget_focus_viewport->set_border_color(accent_color * Color(1, 1, 1, 0.5));
	theme->set_stylebox("FocusViewport", "EditorStyles", style_widget_focus_viewport);
	
	Ref<StyleBoxFlat> style_menu = style_widget->duplicate();
	style_menu->set_draw_center(false);