### 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_launch_pad_movie->set_border_color(accent_color);
	style_launch_pad_movie->set_border_width_all(Math::round(2 * EDSCALE));
	theme->set_stylebox("LaunchPadMovieMode", "EditorStyles", style_launch_pad_movie);
	theme->set_stylebox("MovieWriterButtonNormal", "EditorStyles", make_empty_stylebox(0, 0, 0, 0));
	Ref<StyleBoxFlat> style_write_movie_button = style_widget_pressed->duplicate();
	style_write_movie_button->set_bg_color(accent_color);
	style_write_movie_button->set_corner_radius_all(corner_radius * EDSCALE);
	style_write_movie_button->set_content_margin(SIDE_TOP, 0);
	style_write_movie_button->set_content_margin(SIDE_BOTTOM, 0);
	style_write_movie_button->set_content_margin(SIDE_LEFT, 0);
	style_write_movie_button->set_content_margin(SIDE_RIGHT, 0);
	style_write_movie_button->set_expand_margin(SIDE_RIGHT, 2 * EDSCALE);
	theme->set_stylebox("MovieWriterButtonPressed", "EditorStyles", style_write_movie_button);
	theme->set_stylebox("normal", "MenuButton", style_menu);
	theme->set_stylebox("hover", "MenuButton", style_widget_hover);
	theme->set_stylebox("pressed", "MenuButton", style_menu);
	theme->set_stylebox("focus", "MenuButton", style_menu);
	theme->set_stylebox("disabled", "MenuButton", style_menu);
	theme->set_color("font_color", "MenuButton", font_color);
	theme->set_color("font_hover_color", "MenuButton", font_hover_color);
	theme->set_color("font_hover_pressed_color", "MenuButton", font_hover_pressed_color);
	theme->set_color("font_focus_color", "MenuButton", font_focus_color);
	theme->set_stylebox("MenuHover", "EditorStyles", style_widget_hover);
	
	theme->set_stylebox("normal", "Button", style_widget);
	theme->set_stylebox("hover", "Button", style_widget_hover);
	theme->set_stylebox("pressed", "Button", style_widget_pressed);
	theme->set_stylebox("focus", "Button", style_widget_focus);
	theme->set_stylebox("disabled", "Button", style_widget_disabled);
	theme->set_color("font_color", "Button", font_color);