### 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_menu->set_border_width_all(0);
	theme->set_stylebox("panel", "PanelContainer", style_menu);
	theme->set_stylebox("MenuPanel", "EditorStyles", style_menu);
	
	Ref<StyleBoxFlat> style_canvas_editor_info = make_flat_stylebox(Color(0.0, 0.0, 0.0, 0.2));
	style_canvas_editor_info->set_expand_margin_all(4 * EDSCALE);
	theme->set_stylebox("CanvasItemInfoOverlay", "EditorStyles", style_canvas_editor_info);
	
	
	
	
	Ref<StyleBoxFlat> toolbar_stylebox = memnew(StyleBoxFlat);
	toolbar_stylebox->set_bg_color(accent_color * Color(1, 1, 1, 0.1));
	toolbar_stylebox->set_corner_radius(CORNER_TOP_LEFT, corner_radius * EDSCALE);
	toolbar_stylebox->set_corner_radius(CORNER_TOP_RIGHT, corner_radius * EDSCALE);
	toolbar_stylebox->set_anti_aliased(false);
	
	toolbar_stylebox->set_border_color(accent_color);
	toolbar_stylebox->set_border_width(SIDE_BOTTOM, Math::round(2 * EDSCALE));
	toolbar_stylebox->set_content_margin(SIDE_BOTTOM, 0);
	theme->set_stylebox("ContextualToolbar", "EditorStyles", toolbar_stylebox);
	
	theme->set_stylebox("ScriptEditorPanel", "EditorStyles", make_empty_stylebox(default_margin_size, 0, default_margin_size, default_margin_size));
	theme->set_stylebox("ScriptEditor", "EditorStyles", make_empty_stylebox(0, 0, 0, 0));
	
	Ref<StyleBoxFlat> style_launch_pad = make_flat_stylebox(dark_color_1, 2 * EDSCALE, 0, 2 * EDSCALE, 0, corner_width);
	style_launch_pad->set_corner_radius_all(corner_radius * EDSCALE);
	theme->set_stylebox("LaunchPadNormal", "EditorStyles", style_launch_pad);
	Ref<StyleBoxFlat> style_launch_pad_movie = style_launch_pad->duplicate();
	style_launch_pad_movie->set_bg_color(accent_color * Color(1, 1, 1, 0.1));