### 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_stylebox("breakpoint", "GraphNode", graphsbbreakpoint);
	theme->set_stylebox("position", "GraphNode", graphsbposition);
	theme->set_stylebox("slot", "GraphNode", graphsbslot);
	theme->set_stylebox("state_machine_frame", "GraphNode", smgraphsb);
	theme->set_stylebox("state_machine_selected_frame", "GraphNode", smgraphsbselected);
	Color node_decoration_color = dark_color_1.inverted();
	theme->set_color("title_color", "GraphNode", node_decoration_color);
	node_decoration_color.a = 0.7;
	theme->set_color("close_color", "GraphNode", node_decoration_color);
	theme->set_color("resizer_color", "GraphNode", node_decoration_color);
	theme->set_constant("port_offset", "GraphNode", 0);
	theme->set_constant("title_h_offset", "GraphNode", 12 * EDSCALE);
	theme->set_constant("title_offset", "GraphNode", 21 * EDSCALE);
	theme->set_constant("close_h_offset", "GraphNode", -2 * EDSCALE);
	theme->set_constant("close_offset", "GraphNode", 20 * EDSCALE);
	theme->set_constant("separation", "GraphNode", 1 * EDSCALE);
	theme->set_icon("close", "GraphNode", theme->get_icon(SNAME("GuiCloseCustomizable"), SNAME("EditorIcons")));
	theme->set_icon("resizer", "GraphNode", theme->get_icon(SNAME("GuiResizer"), SNAME("EditorIcons")));
	theme->set_icon("port", "GraphNode", theme->get_icon(SNAME("GuiGraphNodePort"), SNAME("EditorIcons")));
	theme->set_font("title_font", "GraphNode", theme->get_font(SNAME("main_bold_msdf"), SNAME("EditorFonts")));
	
	theme->set_constant("v_separation", "GridContainer", Math::round(widget_default_margin.y - 2 * EDSCALE));
	
	theme->set_icon("folder", "FileDialog", theme->get_icon(SNAME("Folder"), SNAME("EditorIcons")));
	theme->set_icon("parent_folder", "FileDialog", theme->get_icon(SNAME("ArrowUp"), SNAME("EditorIcons")));
	theme->set_icon("back_folder", "FileDialog", theme->get_icon(SNAME("Back"), SNAME("EditorIcons")));
	theme->set_icon("forward_folder", "FileDialog", theme->get_icon(SNAME("Forward"), SNAME("EditorIcons")));
	theme->set_icon("reload", "FileDialog", theme->get_icon(SNAME("Reload"), SNAME("EditorIcons")));
	theme->set_icon("toggle_hidden", "FileDialog", theme->get_icon(SNAME("GuiVisibilityVisible"), SNAME("EditorIcons")));