### 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)

Ref<StyleBoxLine> style(memnew(StyleBoxLine));
	style->set_color(p_color);
	style->set_grow_begin(p_grow_begin);
	style->set_grow_end(p_grow_end);
	style->set_thickness(p_thickness);
	style->set_vertical(p_vertical);
	return style;
}
#ifdef MODULE_SVG_ENABLED
static Ref<ImageTexture> editor_generate_icon(int p_index, float p_scale, float p_saturation, const HashMap<Color, Color> &p_convert_colors = HashMap<Color, Color>()) {
	Ref<Image> img = memnew(Image);
	
	
	
	const bool upsample = !Math::is_equal_approx(Math::round(p_scale), p_scale);
	ImageLoaderSVG img_loader;
	Error err = img_loader.create_image_from_string(img, editor_icons_sources[p_index], p_scale, upsample, p_convert_colors);
	ERR_FAIL_COND_V_MSG(err != OK, Ref<ImageTexture>(), "Failed generating icon, unsupported or invalid SVG data in editor theme.");
	if (p_saturation != 1.0) {
		img->adjust_bcs(1.0, 1.0, p_saturation);
	}
	return ImageTexture::create_from_image(img);
}
#endif
void editor_register_and_generate_icons(Ref<Theme> p_theme, bool p_dark_theme, float p_icon_saturation, int p_thumb_size, bool p_only_thumbs = false) {