### 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)

sub_inspector_bg->set_corner_radius(CORNER_TOP_RIGHT, 0);
		theme->set_stylebox("sub_inspector_bg" + itos(i), "Editor", sub_inspector_bg);
		
		Ref<StyleBoxFlat> bg_color = make_flat_stylebox(si_base_color * Color(0.7, 0.7, 0.7, 0.8), 0, 0, 0, 0, corner_radius);
		bg_color->set_anti_aliased(false);
		bg_color->set_corner_radius(CORNER_BOTTOM_LEFT, 0);
		bg_color->set_corner_radius(CORNER_BOTTOM_RIGHT, 0);
		theme->set_stylebox("sub_inspector_property_bg" + itos(i), "Editor", bg_color);
	}
	theme->set_color("sub_inspector_property_color", "Editor", dark_theme ? Color(1, 1, 1, 1) : Color(0, 0, 0, 1));
	
	theme->set_color("label_color", "EditorSpinSlider", font_color);
	theme->set_color("read_only_label_color", "EditorSpinSlider", font_readonly_color);
	Ref<StyleBoxFlat> editor_spin_label_bg = style_default->duplicate();
	editor_spin_label_bg->set_bg_color(dark_color_3);
	editor_spin_label_bg->set_border_width_all(0);
	theme->set_stylebox("label_bg", "EditorSpinSlider", editor_spin_label_bg);
	
	Ref<StyleBoxFlat> style_property_bg = style_default->duplicate();
	style_property_bg->set_bg_color(highlight_color);
	style_property_bg->set_border_width_all(0);
	Ref<StyleBoxFlat> style_property_child_bg = style_default->duplicate();
	style_property_child_bg->set_bg_color(dark_color_2);
	style_property_child_bg->set_border_width_all(0);
	theme->set_constant("font_offset", "EditorProperty", 8 * EDSCALE);
	theme->set_stylebox("bg_selected", "EditorProperty", style_property_bg);
	theme->set_stylebox("bg", "EditorProperty", Ref<StyleBoxEmpty>(memnew(StyleBoxEmpty)));
	theme->set_stylebox("child_bg", "EditorProperty", style_property_child_bg);
	theme->set_constant("v_separation", "EditorProperty", (extra_spacing + default_margin_size) * EDSCALE);
	theme->set_color("warning_color", "EditorProperty", warning_color);