### 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)

theme->set_stylebox("cursor_unfocused", "Tree", style_tree_cursor);
	theme->set_stylebox("title_button_normal", "Tree", style_tree_title);
	theme->set_stylebox("title_button_hover", "Tree", style_tree_title);
	theme->set_stylebox("title_button_pressed", "Tree", style_tree_title);
	Color prop_category_color = dark_color_1.lerp(mono_color, 0.12);
	Color prop_section_color = dark_color_1.lerp(mono_color, 0.09);
	Color prop_subsection_color = dark_color_1.lerp(mono_color, 0.06);
	theme->set_color("prop_category", "Editor", prop_category_color);
	theme->set_color("prop_section", "Editor", prop_section_color);
	theme->set_color("prop_subsection", "Editor", prop_subsection_color);
	theme->set_color("drop_position_color", "Tree", accent_color);
	
	Ref<StyleBoxFlat> category_bg = style_default->duplicate();
	category_bg->set_bg_color(prop_category_color);
	category_bg->set_border_color(prop_category_color);
	theme->set_stylebox("bg", "EditorInspectorCategory", category_bg);
	
	Ref<StyleBoxFlat> style_itemlist_bg = style_default->duplicate();
	style_itemlist_bg->set_bg_color(dark_color_1);
	if (draw_extra_borders) {
		style_itemlist_bg->set_border_width_all(Math::round(EDSCALE));
		style_itemlist_bg->set_border_color(extra_border_color_2);
	} else {
		style_itemlist_bg->set_border_width_all(border_width);
		style_itemlist_bg->set_border_color(dark_color_3);
	}
	Ref<StyleBoxFlat> style_itemlist_cursor = style_default->duplicate();
	style_itemlist_cursor->set_draw_center(false);
	style_itemlist_cursor->set_border_width_all(border_width);
	style_itemlist_cursor->set_border_color(highlight_color);