### 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)

if (is_fixed && p_property.name == "vertical") {
		p_property.usage = PROPERTY_USAGE_NONE;
	}
}
void SplitContainer::_update_theme_item_cache() {
	Container::_update_theme_item_cache();
	theme_cache.separation = get_theme_constant(SNAME("separation"));
	theme_cache.minimum_grab_thickness = get_theme_constant(SNAME("minimum_grab_thickness"));
	theme_cache.autohide = get_theme_constant(SNAME("autohide"));
	theme_cache.grabber_icon = get_theme_icon(SNAME("grabber"));
	theme_cache.grabber_icon_h = get_theme_icon(SNAME("h_grabber"));
	theme_cache.grabber_icon_v = get_theme_icon(SNAME("v_grabber"));
}
void SplitContainer::_notification(int p_what) {
	switch (p_what) {
		case NOTIFICATION_TRANSLATION_CHANGED:
		case NOTIFICATION_LAYOUT_DIRECTION_CHANGED: {
			queue_sort();
		} break;
		case NOTIFICATION_SORT_CHILDREN: {
			_resort();
		} break;
		case NOTIFICATION_THEME_CHANGED: {
			update_minimum_size();
		} break;
	}
}
void SplitContainer::set_split_offset(int p_offset) {