### 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_color("caret_color", "LineEdit", font_color);
	theme->set_color("selection_color", "LineEdit", selection_color);
	theme->set_color("clear_button_color", "LineEdit", font_color);
	theme->set_color("clear_button_color_pressed", "LineEdit", accent_color);
	
	theme->set_stylebox("normal", "TextEdit", style_line_edit);
	theme->set_stylebox("focus", "TextEdit", style_widget_focus);
	theme->set_stylebox("read_only", "TextEdit", style_line_edit_disabled);
	theme->set_icon("tab", "TextEdit", theme->get_icon(SNAME("GuiTab"), SNAME("EditorIcons")));
	theme->set_icon("space", "TextEdit", theme->get_icon(SNAME("GuiSpace"), SNAME("EditorIcons")));
	theme->set_color("font_color", "TextEdit", font_color);
	theme->set_color("font_readonly_color", "TextEdit", font_readonly_color);
	theme->set_color("font_placeholder_color", "TextEdit", font_placeholder_color);
	theme->set_color("caret_color", "TextEdit", font_color);
	theme->set_color("selection_color", "TextEdit", selection_color);
	theme->set_constant("line_spacing", "TextEdit", 4 * EDSCALE);
	theme->set_icon("h_grabber", "SplitContainer", theme->get_icon(SNAME("GuiHsplitter"), SNAME("EditorIcons")));
	theme->set_icon("v_grabber", "SplitContainer", theme->get_icon(SNAME("GuiVsplitter"), SNAME("EditorIcons")));
	theme->set_icon("grabber", "VSplitContainer", theme->get_icon(SNAME("GuiVsplitter"), SNAME("EditorIcons")));
	theme->set_icon("grabber", "HSplitContainer", theme->get_icon(SNAME("GuiHsplitter"), SNAME("EditorIcons")));
	theme->set_constant("separation", "HSplitContainer", default_margin_size * 2 * EDSCALE);
	theme->set_constant("separation", "VSplitContainer", default_margin_size * 2 * EDSCALE);
	theme->set_constant("minimum_grab_thickness", "HSplitContainer", 6 * EDSCALE);
	theme->set_constant("minimum_grab_thickness", "VSplitContainer", 6 * EDSCALE);
	
	theme->set_constant("separation", "BoxContainer", default_margin_size * EDSCALE);
	theme->set_constant("separation", "HBoxContainer", default_margin_size * EDSCALE);
	theme->set_constant("separation", "VBoxContainer", default_margin_size * EDSCALE);
	theme->set_constant("margin_left", "MarginContainer", 0);
	theme->set_constant("margin_top", "MarginContainer", 0);