### 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("panel", "ItemList", style_itemlist_bg);
	theme->set_stylebox("focus", "ItemList", style_widget_focus);
	theme->set_stylebox("cursor", "ItemList", style_itemlist_cursor);
	theme->set_stylebox("cursor_unfocused", "ItemList", style_itemlist_cursor);
	theme->set_stylebox("selected_focus", "ItemList", style_tree_focus);
	theme->set_stylebox("selected", "ItemList", style_tree_selected);
	theme->set_color("font_color", "ItemList", font_color);
	theme->set_color("font_selected_color", "ItemList", mono_color);
	theme->set_color("guide_color", "ItemList", guide_color);
	theme->set_constant("v_separation", "ItemList", force_even_vsep * 0.5 * EDSCALE);
	theme->set_constant("h_separation", "ItemList", 6 * EDSCALE);
	theme->set_constant("icon_margin", "ItemList", 6 * EDSCALE);
	theme->set_constant("line_separation", "ItemList", 3 * EDSCALE);
	
	Ref<StyleBoxFlat> style_tabbar_background = make_flat_stylebox(dark_color_1, 0, 0, 0, 0, corner_radius * EDSCALE);
	style_tabbar_background->set_corner_radius(CORNER_BOTTOM_LEFT, 0);
	style_tabbar_background->set_corner_radius(CORNER_BOTTOM_RIGHT, 0);
	theme->set_stylebox("tabbar_background", "TabContainer", style_tabbar_background);
	theme->set_stylebox("tab_selected", "TabContainer", style_tab_selected);
	theme->set_stylebox("tab_unselected", "TabContainer", style_tab_unselected);
	theme->set_stylebox("tab_disabled", "TabContainer", style_tab_disabled);
	theme->set_stylebox("tab_selected", "TabBar", style_tab_selected);
	theme->set_stylebox("tab_unselected", "TabBar", style_tab_unselected);
	theme->set_stylebox("tab_disabled", "TabBar", style_tab_disabled);
	theme->set_stylebox("button_pressed", "TabBar", style_menu);
	theme->set_stylebox("button_highlight", "TabBar", style_menu);
	theme->set_color("font_selected_color", "TabContainer", font_color);
	theme->set_color("font_unselected_color", "TabContainer", font_disabled_color);
	theme->set_color("font_selected_color", "TabBar", font_color);
	theme->set_color("font_unselected_color", "TabBar", font_disabled_color);