### 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)

Size2i minimum;
	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;
	for (int i = 0; i < 2; i++) {
		if (!_getch(i)) {
			break;
		}
		if (i == 1) {
			if (vertical) {
				minimum.height += sep;
			} else {
				minimum.width += sep;
			}
		}
		Size2 ms = _getch(i)->get_combined_minimum_size();
		if (vertical) {
			minimum.height += ms.height;
			minimum.width = MAX(minimum.width, ms.width);
		} else {
			minimum.width += ms.width;
			minimum.height = MAX(minimum.height, ms.height);
		}
	}
	return minimum;
}
void SplitContainer::_validate_property(PropertyInfo &p_property) const {