### 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_tooltip->set_shadow_size(0);
	style_tooltip->set_content_margin_all(default_margin_size * EDSCALE * 0.5);
	style_tooltip->set_bg_color(dark_color_3 * Color(0.8, 0.8, 0.8, 0.9));
	style_tooltip->set_border_width_all(0);
	theme->set_color("font_color", "TooltipLabel", font_hover_color);
	theme->set_color("font_shadow_color", "TooltipLabel", Color(0, 0, 0, 0));
	theme->set_stylebox("panel", "TooltipPanel", style_tooltip);
	
	theme->set_stylebox("panel", "PopupPanel", style_popup);
	Ref<StyleBoxFlat> control_editor_popup_style = style_popup->duplicate();
	control_editor_popup_style->set_shadow_size(0);
	control_editor_popup_style->set_content_margin(SIDE_LEFT, default_margin_size * EDSCALE);
	control_editor_popup_style->set_content_margin(SIDE_TOP, default_margin_size * EDSCALE);
	control_editor_popup_style->set_content_margin(SIDE_RIGHT, default_margin_size * EDSCALE);
	control_editor_popup_style->set_content_margin(SIDE_BOTTOM, default_margin_size * EDSCALE);
	control_editor_popup_style->set_border_width_all(0);
	theme->set_stylebox("panel", "ControlEditorPopupPanel", control_editor_popup_style);
	theme->set_type_variation("ControlEditorPopupPanel", "PopupPanel");
	
	theme->set_icon("updown", "SpinBox", theme->get_icon(SNAME("GuiSpinboxUpdown"), SNAME("EditorIcons")));
	theme->set_icon("updown_disabled", "SpinBox", theme->get_icon(SNAME("GuiSpinboxUpdownDisabled"), SNAME("EditorIcons")));
	
	theme->set_stylebox("background", "ProgressBar", make_stylebox(theme->get_icon(SNAME("GuiProgressBar"), SNAME("EditorIcons")), 4, 4, 4, 4, 0, 0, 0, 0));
	theme->set_stylebox("fill", "ProgressBar", make_stylebox(theme->get_icon(SNAME("GuiProgressFill"), SNAME("EditorIcons")), 6, 6, 6, 6, 2, 1, 2, 1));
	theme->set_color("font_color", "ProgressBar", font_color);
	
	theme->set_stylebox("bg", "GraphEdit", style_tree_bg);
	if (dark_theme) {
		theme->set_color("grid_major", "GraphEdit", Color(1.0, 1.0, 1.0, 0.15));
		theme->set_color("grid_minor", "GraphEdit", Color(1.0, 1.0, 1.0, 0.07));