### 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_color("kbd_bg_color", "EditorHelp", dark_color_1);
	theme->set_color("param_bg_color", "EditorHelp", dark_color_1);
	theme->set_constant("line_separation", "EditorHelp", Math::round(6 * EDSCALE));
	theme->set_constant("table_h_separation", "EditorHelp", 16 * EDSCALE);
	theme->set_constant("table_v_separation", "EditorHelp", 6 * EDSCALE);
	theme->set_constant("text_highlight_h_padding", "EditorHelp", 1 * EDSCALE);
	theme->set_constant("text_highlight_v_padding", "EditorHelp", 2 * EDSCALE);
	
	theme->set_stylebox("panel", "Panel", make_flat_stylebox(dark_color_1, 6, 4, 6, 4, corner_width));
	theme->set_stylebox("PanelForeground", "EditorStyles", style_default);
	
	theme->set_stylebox("normal", "Label", style_empty);
	theme->set_color("font_color", "Label", font_color);
	theme->set_color("font_shadow_color", "Label", Color(0, 0, 0, 0));
	theme->set_constant("shadow_offset_x", "Label", 1 * EDSCALE);
	theme->set_constant("shadow_offset_y", "Label", 1 * EDSCALE);
	theme->set_constant("shadow_outline_size", "Label", 1 * EDSCALE);
	theme->set_constant("line_spacing", "Label", 3 * EDSCALE);
	
	theme->set_stylebox("focus", "LinkButton", style_empty);
	theme->set_color("font_color", "LinkButton", font_color);
	theme->set_color("font_hover_color", "LinkButton", font_hover_color);
	theme->set_color("font_hover_pressed_color", "LinkButton", font_hover_pressed_color);
	theme->set_color("font_focus_color", "LinkButton", font_focus_color);
	theme->set_color("font_pressed_color", "LinkButton", accent_color);
	theme->set_color("font_disabled_color", "LinkButton", font_disabled_color);
	
	
	
	Ref<StyleBoxFlat> style_tooltip = style_popup->duplicate();