### 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("font_hover_color", "Button", font_hover_color);
	theme->set_color("font_hover_pressed_color", "Button", font_hover_pressed_color);
	theme->set_color("font_focus_color", "Button", font_focus_color);
	theme->set_color("font_pressed_color", "Button", accent_color);
	theme->set_color("font_disabled_color", "Button", font_disabled_color);
	theme->set_color("icon_normal_color", "Button", icon_normal_color);
	theme->set_color("icon_hover_color", "Button", icon_hover_color);
	theme->set_color("icon_focus_color", "Button", icon_focus_color);
	theme->set_color("icon_pressed_color", "Button", icon_pressed_color);
	theme->set_color("icon_disabled_color", "Button", icon_disabled_color);
	const float ACTION_BUTTON_EXTRA_MARGIN = 32 * EDSCALE;
	theme->set_type_variation("InspectorActionButton", "Button");
	Color color_inspector_action = dark_color_1.lerp(mono_color, 0.12);
	color_inspector_action.a = 0.5;
	Ref<StyleBoxFlat> style_inspector_action = style_widget->duplicate();
	style_inspector_action->set_bg_color(color_inspector_action);
	style_inspector_action->set_content_margin(SIDE_RIGHT, ACTION_BUTTON_EXTRA_MARGIN);
	theme->set_stylebox("normal", "InspectorActionButton", style_inspector_action);
	style_inspector_action = style_widget_hover->duplicate();
	style_inspector_action->set_content_margin(SIDE_RIGHT, ACTION_BUTTON_EXTRA_MARGIN);
	theme->set_stylebox("hover", "InspectorActionButton", style_inspector_action);
	style_inspector_action = style_widget_pressed->duplicate();
	style_inspector_action->set_content_margin(SIDE_RIGHT, ACTION_BUTTON_EXTRA_MARGIN);
	theme->set_stylebox("pressed", "InspectorActionButton", style_inspector_action);
	style_inspector_action = style_widget_disabled->duplicate();
	style_inspector_action->set_content_margin(SIDE_RIGHT, ACTION_BUTTON_EXTRA_MARGIN);
	theme->set_stylebox("disabled", "InspectorActionButton", style_inspector_action);
	theme->set_constant("h_separation", "InspectorActionButton", ACTION_BUTTON_EXTRA_MARGIN);
	
	theme->set_type_variation("EditorLogFilterButton", "Button");