### 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_color("icon_pressed_color", "EditorLogFilterButton", icon_normal_color);
	
	theme->set_color("icon_normal_color", "EditorLogFilterButton", icon_disabled_color);
	
	
	Ref<StyleBoxFlat> editor_log_button_pressed = style_widget_pressed->duplicate();
	editor_log_button_pressed->set_border_width(SIDE_BOTTOM, 2 * EDSCALE);
	editor_log_button_pressed->set_border_color(accent_color);
	theme->set_stylebox("pressed", "EditorLogFilterButton", editor_log_button_pressed);
	
	theme->set_stylebox("normal", "MenuBar", style_widget);
	theme->set_stylebox("hover", "MenuBar", style_widget_hover);
	theme->set_stylebox("pressed", "MenuBar", style_widget_pressed);
	theme->set_stylebox("focus", "MenuBar", style_widget_focus);
	theme->set_stylebox("disabled", "MenuBar", style_widget_disabled);
	theme->set_color("font_color", "MenuBar", font_color);
	theme->set_color("font_hover_color", "MenuBar", font_hover_color);
	theme->set_color("font_hover_pressed_color", "MenuBar", font_hover_pressed_color);
	theme->set_color("font_focus_color", "MenuBar", font_focus_color);
	theme->set_color("font_pressed_color", "MenuBar", accent_color);
	theme->set_color("font_disabled_color", "MenuBar", font_disabled_color);
	theme->set_color("icon_normal_color", "MenuBar", icon_normal_color);
	theme->set_color("icon_hover_color", "MenuBar", icon_hover_color);
	theme->set_color("icon_focus_color", "MenuBar", icon_focus_color);
	theme->set_color("icon_pressed_color", "MenuBar", icon_pressed_color);
	theme->set_color("icon_disabled_color", "MenuBar", icon_disabled_color);
	
	Ref<StyleBoxFlat> style_option_button_focus = style_widget_focus->duplicate();
	Ref<StyleBoxFlat> style_option_button_normal = style_widget->duplicate();