### 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("property_color", "EditorProperty", property_color);
	theme->set_color("readonly_color", "EditorProperty", readonly_color);
	theme->set_color("readonly_warning_color", "EditorProperty", readonly_warning_color);
	Ref<StyleBoxFlat> style_property_group_note = style_default->duplicate();
	Color property_group_note_color = accent_color;
	property_group_note_color.a = 0.1;
	style_property_group_note->set_bg_color(property_group_note_color);
	theme->set_stylebox("bg_group_note", "EditorProperty", style_property_group_note);
	
	Color inspector_section_color = font_color.lerp(Color(0.5, 0.5, 0.5), 0.35);
	theme->set_color("font_color", "EditorInspectorSection", inspector_section_color);
	Color inspector_indent_color = accent_color;
	inspector_indent_color.a = 0.2;
	Ref<StyleBoxFlat> inspector_indent_style = make_flat_stylebox(inspector_indent_color, 2.0 * EDSCALE, 0, 2.0 * EDSCALE, 0);
	theme->set_stylebox("indent_box", "EditorInspectorSection", inspector_indent_style);
	theme->set_constant("indent_size", "EditorInspectorSection", 6.0 * EDSCALE);
	theme->set_constant("inspector_margin", "Editor", 12 * EDSCALE);
	
	Ref<StyleBoxFlat> style_tree_bg = style_default->duplicate();
	
	style_tree_bg->set_bg_color(dark_color_1.lerp(dark_color_2, 0.5));
	if (draw_extra_borders) {
		style_tree_bg->set_border_width_all(Math::round(EDSCALE));
		style_tree_bg->set_border_color(extra_border_color_2);
	} else {
		style_tree_bg->set_border_color(dark_color_3);
	}
	theme->set_stylebox("panel", "Tree", style_tree_bg);
	
	theme->set_icon("checked", "Tree", theme->get_icon(SNAME("GuiChecked"), SNAME("EditorIcons")));