### 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)

Control *first = _getch(0);
	Control *second = _getch(1);
	
	bool first_expanded = (vertical ? first->get_v_size_flags() : first->get_h_size_flags()) & SIZE_EXPAND;
	bool second_expanded = (vertical ? second->get_v_size_flags() : second->get_h_size_flags()) & SIZE_EXPAND;
	
	int axis = vertical ? 1 : 0;
	int size = get_size()[axis];
	int ms_first = first->get_combined_minimum_size()[axis];
	int ms_second = second->get_combined_minimum_size()[axis];
	
	Ref<Texture2D> g = _get_grabber_icon();
	int sep = (dragger_visibility != DRAGGER_HIDDEN_COLLAPSED) ? MAX(theme_cache.separation, vertical ? g->get_height() : g->get_width()) : 0;
	
	int wished_middle_sep = 0;
	int split_offset_with_collapse = 0;
	if (!collapsed) {
		split_offset_with_collapse = split_offset;
	}
	if (first_expanded && second_expanded) {
		float ratio = first->get_stretch_ratio() / (first->get_stretch_ratio() + second->get_stretch_ratio());
		wished_middle_sep = size * ratio - sep / 2 + split_offset_with_collapse;
	} else if (first_expanded) {
		wished_middle_sep = size - sep + split_offset_with_collapse;