### 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_constant("margin_right", "MarginContainer", 0);
	theme->set_constant("margin_bottom", "MarginContainer", 0);
	theme->set_constant("h_separation", "GridContainer", default_margin_size * EDSCALE);
	theme->set_constant("v_separation", "GridContainer", default_margin_size * EDSCALE);
	theme->set_constant("h_separation", "FlowContainer", default_margin_size * EDSCALE);
	theme->set_constant("v_separation", "FlowContainer", default_margin_size * EDSCALE);
	theme->set_constant("h_separation", "HFlowContainer", default_margin_size * EDSCALE);
	theme->set_constant("v_separation", "HFlowContainer", default_margin_size * EDSCALE);
	theme->set_constant("h_separation", "VFlowContainer", default_margin_size * EDSCALE);
	theme->set_constant("v_separation", "VFlowContainer", default_margin_size * EDSCALE);
	
	theme->set_type_variation("MarginContainer4px", "MarginContainer");
	theme->set_constant("margin_left", "MarginContainer4px", 4 * EDSCALE);
	theme->set_constant("margin_top", "MarginContainer4px", 4 * EDSCALE);
	theme->set_constant("margin_right", "MarginContainer4px", 4 * EDSCALE);
	theme->set_constant("margin_bottom", "MarginContainer4px", 4 * EDSCALE);
	
	
	Ref<StyleBoxFlat> style_window_title = style_default->duplicate();
	style_window_title->set_corner_radius(CORNER_TOP_LEFT, 0);
	style_window_title->set_corner_radius(CORNER_TOP_RIGHT, 0);
	
	style_window_title->set_expand_margin(SIDE_BOTTOM, 2 * EDSCALE);
	Ref<StyleBoxFlat> style_window = style_popup->duplicate();
	style_window->set_border_color(base_color);
	style_window->set_border_width(SIDE_TOP, 24 * EDSCALE);
	style_window->set_expand_margin(SIDE_TOP, 24 * EDSCALE);
	theme->set_stylebox("embedded_border", "Window", style_window);
	theme->set_color("title_color", "Window", font_color);
	theme->set_icon("close", "Window", theme->get_icon(SNAME("GuiClose"), SNAME("EditorIcons")));