### 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_panel_invisible_top->set_content_margin(SIDE_TOP, 0);
	theme->set_stylebox("BottomPanelDebuggerOverride", "EditorStyles", style_panel_invisible_top);
	
	Ref<StyleBoxFlat> style_line_edit = style_widget->duplicate();
	
	
	style_line_edit->set_content_margin(SIDE_TOP, style_line_edit->get_content_margin(SIDE_TOP) - 1 * EDSCALE);
	
	style_tab_selected->set_corner_radius(CORNER_BOTTOM_LEFT, 0);
	style_tab_selected->set_corner_radius(CORNER_BOTTOM_RIGHT, 0);
	if (draw_extra_borders) {
		style_line_edit->set_border_width_all(Math::round(EDSCALE));
		style_line_edit->set_border_color(extra_border_color_1);
	} else {
		
		
		style_line_edit->set_border_width(SIDE_BOTTOM, Math::round(2 * EDSCALE));
		style_line_edit->set_border_color(dark_color_2);
	}
	Ref<StyleBoxFlat> style_line_edit_disabled = style_line_edit->duplicate();
	style_line_edit_disabled->set_border_color(disabled_color);
	style_line_edit_disabled->set_bg_color(disabled_bg_color);
	theme->set_stylebox("normal", "LineEdit", style_line_edit);
	theme->set_stylebox("focus", "LineEdit", style_widget_focus);
	theme->set_stylebox("read_only", "LineEdit", style_line_edit_disabled);
	theme->set_icon("clear", "LineEdit", theme->get_icon(SNAME("GuiClose"), SNAME("EditorIcons")));
	theme->set_color("font_color", "LineEdit", font_color);
	theme->set_color("font_selected_color", "LineEdit", mono_color);
	theme->set_color("font_uneditable_color", "LineEdit", font_readonly_color);
	theme->set_color("font_placeholder_color", "LineEdit", font_placeholder_color);