### 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_constant("draw_relationship_lines", "Tree", relationship_line_opacity >= 0.01);
	theme->set_constant("relationship_line_width", "Tree", relationship_line_width);
	theme->set_constant("parent_hl_line_width", "Tree", relationship_line_width * 2);
	theme->set_constant("children_hl_line_width", "Tree", relationship_line_width);
	theme->set_constant("parent_hl_line_margin", "Tree", relationship_line_width * 3);
	theme->set_color("relationship_line_color", "Tree", relationship_line_color);
	theme->set_color("parent_hl_line_color", "Tree", parent_line_color);
	theme->set_color("children_hl_line_color", "Tree", children_line_color);
	Ref<StyleBoxFlat> style_tree_btn = style_default->duplicate();
	style_tree_btn->set_bg_color(highlight_color);
	style_tree_btn->set_border_width_all(0);
	theme->set_stylebox("button_pressed", "Tree", style_tree_btn);
	Ref<StyleBoxFlat> style_tree_hover = style_default->duplicate();
	style_tree_hover->set_bg_color(highlight_color * Color(1, 1, 1, 0.4));
	style_tree_hover->set_border_width_all(0);
	theme->set_stylebox("hover", "Tree", style_tree_hover);
	Ref<StyleBoxFlat> style_tree_focus = style_default->duplicate();
	style_tree_focus->set_bg_color(highlight_color);
	style_tree_focus->set_border_width_all(0);
	theme->set_stylebox("selected_focus", "Tree", style_tree_focus);
	Ref<StyleBoxFlat> style_tree_selected = style_tree_focus->duplicate();
	theme->set_stylebox("selected", "Tree", style_tree_selected);
	Ref<StyleBoxFlat> style_tree_cursor = style_default->duplicate();
	style_tree_cursor->set_draw_center(false);
	style_tree_cursor->set_border_width_all(MAX(1, border_width));
	style_tree_cursor->set_border_color(contrast_color_1);
	Ref<StyleBoxFlat> style_tree_title = style_default->duplicate();
	style_tree_title->set_bg_color(dark_color_3);
	style_tree_title->set_border_width_all(0);
	theme->set_stylebox("cursor", "Tree", style_tree_cursor);