### 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)

Ref<StyleBoxFlat> style_option_button_hover = style_widget_hover->duplicate();
	Ref<StyleBoxFlat> style_option_button_pressed = style_widget_pressed->duplicate();
	Ref<StyleBoxFlat> style_option_button_disabled = style_widget_disabled->duplicate();
	style_option_button_focus->set_content_margin(SIDE_RIGHT, 4 * EDSCALE);
	style_option_button_normal->set_content_margin(SIDE_RIGHT, 4 * EDSCALE);
	style_option_button_hover->set_content_margin(SIDE_RIGHT, 4 * EDSCALE);
	style_option_button_pressed->set_content_margin(SIDE_RIGHT, 4 * EDSCALE);
	style_option_button_disabled->set_content_margin(SIDE_RIGHT, 4 * EDSCALE);
	theme->set_stylebox("focus", "OptionButton", style_option_button_focus);
	theme->set_stylebox("normal", "OptionButton", style_widget);
	theme->set_stylebox("hover", "OptionButton", style_widget_hover);
	theme->set_stylebox("pressed", "OptionButton", style_widget_pressed);
	theme->set_stylebox("disabled", "OptionButton", style_widget_disabled);
	theme->set_stylebox("normal_mirrored", "OptionButton", style_option_button_normal);
	theme->set_stylebox("hover_mirrored", "OptionButton", style_option_button_hover);
	theme->set_stylebox("pressed_mirrored", "OptionButton", style_option_button_pressed);
	theme->set_stylebox("disabled_mirrored", "OptionButton", style_option_button_disabled);
	theme->set_color("font_color", "OptionButton", font_color);
	theme->set_color("font_hover_color", "OptionButton", font_hover_color);
	theme->set_color("font_hover_pressed_color", "OptionButton", font_hover_pressed_color);
	theme->set_color("font_focus_color", "OptionButton", font_focus_color);
	theme->set_color("font_pressed_color", "OptionButton", accent_color);
	theme->set_color("font_disabled_color", "OptionButton", font_disabled_color);
	theme->set_color("icon_normal_color", "OptionButton", icon_normal_color);
	theme->set_color("icon_hover_color", "OptionButton", icon_hover_color);
	theme->set_color("icon_focus_color", "OptionButton", icon_focus_color);
	theme->set_color("icon_pressed_color", "OptionButton", icon_pressed_color);
	theme->set_color("icon_disabled_color", "OptionButton", icon_disabled_color);
	theme->set_icon("arrow", "OptionButton", theme->get_icon(SNAME("GuiOptionArrow"), SNAME("EditorIcons")));
	theme->set_constant("arrow_margin", "OptionButton", widget_default_margin.x - 2 * EDSCALE);