### 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_icon("indeterminate", "Tree", theme->get_icon(SNAME("GuiIndeterminate"), SNAME("EditorIcons")));
	theme->set_icon("unchecked", "Tree", theme->get_icon(SNAME("GuiUnchecked"), SNAME("EditorIcons")));
	theme->set_icon("arrow", "Tree", theme->get_icon(SNAME("GuiTreeArrowDown"), SNAME("EditorIcons")));
	theme->set_icon("arrow_collapsed", "Tree", theme->get_icon(SNAME("GuiTreeArrowRight"), SNAME("EditorIcons")));
	theme->set_icon("arrow_collapsed_mirrored", "Tree", theme->get_icon(SNAME("GuiTreeArrowLeft"), SNAME("EditorIcons")));
	theme->set_icon("updown", "Tree", theme->get_icon(SNAME("GuiTreeUpdown"), SNAME("EditorIcons")));
	theme->set_icon("select_arrow", "Tree", theme->get_icon(SNAME("GuiDropdown"), SNAME("EditorIcons")));
	theme->set_stylebox("focus", "Tree", style_widget_focus);
	theme->set_stylebox("custom_button", "Tree", make_empty_stylebox());
	theme->set_stylebox("custom_button_pressed", "Tree", make_empty_stylebox());
	theme->set_stylebox("custom_button_hover", "Tree", style_widget);
	theme->set_color("custom_button_font_highlight", "Tree", font_hover_color);
	theme->set_color("font_color", "Tree", font_color);
	theme->set_color("font_selected_color", "Tree", mono_color);
	theme->set_color("title_button_color", "Tree", font_color);
	theme->set_color("drop_position_color", "Tree", accent_color);
	theme->set_constant("v_separation", "Tree", widget_default_margin.y - EDSCALE);
	theme->set_constant("h_separation", "Tree", 6 * EDSCALE);
	theme->set_constant("guide_width", "Tree", border_width);
	theme->set_constant("item_margin", "Tree", 3 * default_margin_size * EDSCALE);
	theme->set_constant("button_margin", "Tree", default_margin_size * EDSCALE);
	theme->set_constant("scroll_border", "Tree", 40 * EDSCALE);
	theme->set_constant("scroll_speed", "Tree", 12);
	const Color guide_color = mono_color * Color(1, 1, 1, 0.05);
	Color relationship_line_color = mono_color * Color(1, 1, 1, relationship_line_opacity);
	theme->set_constant("draw_guides", "Tree", relationship_line_opacity < 0.01);
	theme->set_color("guide_color", "Tree", guide_color);
	int relationship_line_width = 1;
	Color parent_line_color = mono_color * Color(1, 1, 1, CLAMP(relationship_line_opacity + 0.45, 0.0, 1.0));
	Color children_line_color = mono_color * Color(1, 1, 1, CLAMP(relationship_line_opacity + 0.25, 0.0, 1.0));