### 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("modulate_arrow", "OptionButton", true);
	theme->set_constant("h_separation", "OptionButton", 4 * EDSCALE);
	
	theme->set_stylebox("normal", "CheckButton", style_menu);
	theme->set_stylebox("pressed", "CheckButton", style_menu);
	theme->set_stylebox("disabled", "CheckButton", style_menu);
	theme->set_stylebox("hover", "CheckButton", style_menu);
	theme->set_stylebox("hover_pressed", "CheckButton", style_menu);
	theme->set_icon("checked", "CheckButton", theme->get_icon(SNAME("GuiToggleOn"), SNAME("EditorIcons")));
	theme->set_icon("checked_disabled", "CheckButton", theme->get_icon(SNAME("GuiToggleOnDisabled"), SNAME("EditorIcons")));
	theme->set_icon("unchecked", "CheckButton", theme->get_icon(SNAME("GuiToggleOff"), SNAME("EditorIcons")));
	theme->set_icon("unchecked_disabled", "CheckButton", theme->get_icon(SNAME("GuiToggleOffDisabled"), SNAME("EditorIcons")));
	theme->set_icon("checked_mirrored", "CheckButton", theme->get_icon(SNAME("GuiToggleOnMirrored"), SNAME("EditorIcons")));
	theme->set_icon("checked_disabled_mirrored", "CheckButton", theme->get_icon(SNAME("GuiToggleOnDisabledMirrored"), SNAME("EditorIcons")));
	theme->set_icon("unchecked_mirrored", "CheckButton", theme->get_icon(SNAME("GuiToggleOffMirrored"), SNAME("EditorIcons")));
	theme->set_icon("unchecked_disabled_mirrored", "CheckButton", theme->get_icon(SNAME("GuiToggleOffDisabledMirrored"), SNAME("EditorIcons")));
	theme->set_color("font_color", "CheckButton", font_color);
	theme->set_color("font_hover_color", "CheckButton", font_hover_color);
	theme->set_color("font_hover_pressed_color", "CheckButton", font_hover_pressed_color);
	theme->set_color("font_focus_color", "CheckButton", font_focus_color);
	theme->set_color("font_pressed_color", "CheckButton", accent_color);
	theme->set_color("font_disabled_color", "CheckButton", font_disabled_color);
	theme->set_color("icon_normal_color", "CheckButton", icon_normal_color);
	theme->set_color("icon_hover_color", "CheckButton", icon_hover_color);
	theme->set_color("icon_focus_color", "CheckButton", icon_focus_color);
	theme->set_color("icon_pressed_color", "CheckButton", icon_pressed_color);
	theme->set_color("icon_disabled_color", "CheckButton", icon_disabled_color);
	theme->set_constant("h_separation", "CheckButton", 8 * EDSCALE);
	theme->set_constant("check_v_offset", "CheckButton", 0 * EDSCALE);