### 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_stylebox("bg", "AssetLib", style_empty);
	theme->set_stylebox("panel", "AssetLib", style_content_panel);
	theme->set_color("status_color", "AssetLib", Color(0.5, 0.5, 0.5));
	theme->set_icon("dismiss", "AssetLib", theme->get_icon(SNAME("Close"), SNAME("EditorIcons")));
	
	theme->set_color("preview_picker_overlay_color", "ThemeEditor", Color(0.1, 0.1, 0.1, 0.25));
	Color theme_preview_picker_bg_color = accent_color;
	theme_preview_picker_bg_color.a = 0.2;
	Ref<StyleBoxFlat> theme_preview_picker_sb = make_flat_stylebox(theme_preview_picker_bg_color, 0, 0, 0, 0);
	theme_preview_picker_sb->set_border_color(accent_color);
	theme_preview_picker_sb->set_border_width_all(1.0 * EDSCALE);
	theme->set_stylebox("preview_picker_overlay", "ThemeEditor", theme_preview_picker_sb);
	Color theme_preview_picker_label_bg_color = accent_color;
	theme_preview_picker_label_bg_color.set_v(0.5);
	Ref<StyleBoxFlat> theme_preview_picker_label_sb = make_flat_stylebox(theme_preview_picker_label_bg_color, 4.0, 1.0, 4.0, 3.0);
	theme->set_stylebox("preview_picker_label", "ThemeEditor", theme_preview_picker_label_sb);
	
	
	Ref<StyleBoxFlat> style_dictionary_add_item = make_flat_stylebox(prop_subsection_color, 0, 4, 0, 4, corner_radius);
	style_dictionary_add_item->set_expand_margin(SIDE_LEFT, 4 * EDSCALE);
	style_dictionary_add_item->set_expand_margin(SIDE_RIGHT, 4 * EDSCALE);
	theme->set_stylebox("DictionaryAddItem", "EditorStyles", style_dictionary_add_item);
	Ref<StyleBoxEmpty> vshader_label_style = make_empty_stylebox(2, 1, 2, 1);
	theme->set_stylebox("label_style", "VShaderEditor", vshader_label_style);
	
	
	const Color dim_color = Color(font_color.r, font_color.g, font_color.b, 0.5);
	const float mono_value = mono_color.r;
	const Color alpha1 = Color(mono_value, mono_value, mono_value, 0.07);