### 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_icon("increment_pressed", "HScrollBar", empty_icon);
	theme->set_icon("decrement", "HScrollBar", empty_icon);
	theme->set_icon("decrement_highlight", "HScrollBar", empty_icon);
	theme->set_icon("decrement_pressed", "HScrollBar", empty_icon);
	
	if (enable_touchscreen_touch_area) {
		theme->set_stylebox("scroll", "VScrollBar", make_line_stylebox(separator_color, 50, 1, 1, true));
	} else {
		theme->set_stylebox("scroll", "VScrollBar", make_stylebox(theme->get_icon(SNAME("GuiScrollBg"), SNAME("EditorIcons")), 5, 5, 5, 5, 1, 1, 1, 1));
	}
	theme->set_stylebox("scroll_focus", "VScrollBar", make_stylebox(theme->get_icon(SNAME("GuiScrollBg"), SNAME("EditorIcons")), 5, 5, 5, 5, 1, 1, 1, 1));
	theme->set_stylebox("grabber", "VScrollBar", make_stylebox(theme->get_icon(SNAME("GuiScrollGrabber"), SNAME("EditorIcons")), 6, 6, 6, 6, 1, 1, 1, 1));
	theme->set_stylebox("grabber_highlight", "VScrollBar", make_stylebox(theme->get_icon(SNAME("GuiScrollGrabberHl"), SNAME("EditorIcons")), 5, 5, 5, 5, 1, 1, 1, 1));
	theme->set_stylebox("grabber_pressed", "VScrollBar", make_stylebox(theme->get_icon(SNAME("GuiScrollGrabberPressed"), SNAME("EditorIcons")), 6, 6, 6, 6, 1, 1, 1, 1));
	theme->set_icon("increment", "VScrollBar", empty_icon);
	theme->set_icon("increment_highlight", "VScrollBar", empty_icon);
	theme->set_icon("increment_pressed", "VScrollBar", empty_icon);
	theme->set_icon("decrement", "VScrollBar", empty_icon);
	theme->set_icon("decrement_highlight", "VScrollBar", empty_icon);
	theme->set_icon("decrement_pressed", "VScrollBar", empty_icon);
	
	theme->set_icon("grabber_highlight", "HSlider", theme->get_icon(SNAME("GuiSliderGrabberHl"), SNAME("EditorIcons")));
	theme->set_icon("grabber", "HSlider", theme->get_icon(SNAME("GuiSliderGrabber"), SNAME("EditorIcons")));
	theme->set_stylebox("slider", "HSlider", make_flat_stylebox(dark_color_3, 0, default_margin_size / 2, 0, default_margin_size / 2, corner_width));
	theme->set_stylebox("grabber_area", "HSlider", make_flat_stylebox(contrast_color_1, 0, default_margin_size / 2, 0, default_margin_size / 2, corner_width));
	theme->set_stylebox("grabber_area_highlight", "HSlider", make_flat_stylebox(contrast_color_1, 0, default_margin_size / 2, 0, default_margin_size / 2));
	theme->set_constant("grabber_offset", "HSlider", 0);
	
	theme->set_icon("grabber", "VSlider", theme->get_icon(SNAME("GuiSliderGrabber"), SNAME("EditorIcons")));
	theme->set_icon("grabber_highlight", "VSlider", theme->get_icon(SNAME("GuiSliderGrabberHl"), SNAME("EditorIcons")));