### 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)

style_popup->set_corner_radius_all(0);
	Ref<StyleBoxLine> style_popup_separator(memnew(StyleBoxLine));
	style_popup_separator->set_color(separator_color);
	style_popup_separator->set_grow_begin(popup_margin_size - MAX(Math::round(EDSCALE), border_width));
	style_popup_separator->set_grow_end(popup_margin_size - MAX(Math::round(EDSCALE), border_width));
	style_popup_separator->set_thickness(MAX(Math::round(EDSCALE), border_width));
	Ref<StyleBoxLine> style_popup_labeled_separator_left(memnew(StyleBoxLine));
	style_popup_labeled_separator_left->set_grow_begin(popup_margin_size - MAX(Math::round(EDSCALE), border_width));
	style_popup_labeled_separator_left->set_color(separator_color);
	style_popup_labeled_separator_left->set_thickness(MAX(Math::round(EDSCALE), border_width));
	Ref<StyleBoxLine> style_popup_labeled_separator_right(memnew(StyleBoxLine));
	style_popup_labeled_separator_right->set_grow_end(popup_margin_size - MAX(Math::round(EDSCALE), border_width));
	style_popup_labeled_separator_right->set_color(separator_color);
	style_popup_labeled_separator_right->set_thickness(MAX(Math::round(EDSCALE), border_width));
	Ref<StyleBoxEmpty> style_empty = make_empty_stylebox(default_margin_size, default_margin_size, default_margin_size, default_margin_size);
	
	Ref<StyleBoxFlat> style_tab_base = style_widget->duplicate();
	style_tab_base->set_border_width_all(0);
	
	
	style_tab_base->set_corner_radius(CORNER_BOTTOM_LEFT, 0);
	style_tab_base->set_corner_radius(CORNER_BOTTOM_RIGHT, 0);
	
	style_tab_base->set_expand_margin(SIDE_LEFT, -border_width);
	style_tab_base->set_content_margin(SIDE_LEFT, widget_default_margin.x + 5 * EDSCALE);
	style_tab_base->set_content_margin(SIDE_RIGHT, widget_default_margin.x + 5 * EDSCALE);
	style_tab_base->set_content_margin(SIDE_BOTTOM, widget_default_margin.y);
	style_tab_base->set_content_margin(SIDE_TOP, widget_default_margin.y);
	Ref<StyleBoxFlat> style_tab_selected = style_tab_base->duplicate();