### 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("checked_disabled", "PopupMenu", theme->get_icon(SNAME("GuiCheckedDisabled"), SNAME("EditorIcons")));
	theme->set_icon("unchecked_disabled", "PopupMenu", theme->get_icon(SNAME("GuiUncheckedDisabled"), SNAME("EditorIcons")));
	theme->set_icon("radio_checked_disabled", "PopupMenu", theme->get_icon(SNAME("GuiRadioCheckedDisabled"), SNAME("EditorIcons")));
	theme->set_icon("radio_unchecked_disabled", "PopupMenu", theme->get_icon(SNAME("GuiRadioUncheckedDisabled"), SNAME("EditorIcons")));
	theme->set_icon("submenu", "PopupMenu", theme->get_icon(SNAME("ArrowRight"), SNAME("EditorIcons")));
	theme->set_icon("submenu_mirrored", "PopupMenu", theme->get_icon(SNAME("ArrowLeft"), SNAME("EditorIcons")));
	theme->set_icon("visibility_hidden", "PopupMenu", theme->get_icon(SNAME("GuiVisibilityHidden"), SNAME("EditorIcons")));
	theme->set_icon("visibility_visible", "PopupMenu", theme->get_icon(SNAME("GuiVisibilityVisible"), SNAME("EditorIcons")));
	theme->set_icon("visibility_xray", "PopupMenu", theme->get_icon(SNAME("GuiVisibilityXray"), SNAME("EditorIcons")));
	
	
	
	const int vsep_base = extra_spacing + default_margin_size + 6;
	const int force_even_vsep = vsep_base + (vsep_base % 2);
	theme->set_constant("v_separation", "PopupMenu", force_even_vsep * EDSCALE);
	theme->set_constant("item_start_padding", "PopupMenu", default_margin_size * 1.5 * EDSCALE);
	theme->set_constant("item_end_padding", "PopupMenu", default_margin_size * 1.5 * EDSCALE);
	
	for (int i = 0; i < 16; i++) {
		Color si_base_color = accent_color;
		float hue_rotate = (i * 2 % 16) / 16.0;
		si_base_color.set_hsv(Math::fmod(float(si_base_color.get_h() + hue_rotate), float(1.0)), si_base_color.get_s(), si_base_color.get_v());
		si_base_color = accent_color.lerp(si_base_color, float(EDITOR_GET("docks/property_editor/subresource_hue_tint")));
		
		Ref<StyleBoxFlat> sub_inspector_bg = style_default->duplicate();
		sub_inspector_bg->set_bg_color(dark_color_1.lerp(si_base_color, 0.08));
		sub_inspector_bg->set_border_width_all(2 * EDSCALE);
		sub_inspector_bg->set_border_color(si_base_color * Color(0.7, 0.7, 0.7, 0.8));
		sub_inspector_bg->set_content_margin_all(4 * EDSCALE);
		sub_inspector_bg->set_corner_radius(CORNER_TOP_LEFT, 0);