### 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("BottomPanel", "EditorStyles", style_bottom_panel);
	
	theme->set_type_variation("TabContainerOdd", "TabContainer");
	Ref<StyleBoxFlat> style_tab_selected_odd = style_tab_selected->duplicate();
	style_tab_selected_odd->set_bg_color(disabled_bg_color);
	theme->set_stylebox("tab_selected", "TabContainerOdd", style_tab_selected_odd);
	Ref<StyleBoxFlat> style_content_panel_odd = style_content_panel->duplicate();
	style_content_panel_odd->set_bg_color(disabled_bg_color);
	theme->set_stylebox("panel", "TabContainerOdd", style_content_panel_odd);
	
	Ref<StyleBoxFlat> style_content_panel_vp = style_content_panel->duplicate();
	style_content_panel_vp->set_content_margin_individual(border_width * 2, default_margin_size * EDSCALE, border_width * 2, border_width * 2);
	theme->set_stylebox("Content", "EditorStyles", style_content_panel_vp);
	
	Ref<StyleBoxFlat> style_theme_preview_tab = style_tab_selected_odd->duplicate();
	style_theme_preview_tab->set_expand_margin(SIDE_BOTTOM, 5 * EDSCALE);
	theme->set_stylebox("ThemeEditorPreviewFG", "EditorStyles", style_theme_preview_tab);
	Ref<StyleBoxFlat> style_theme_preview_bg_tab = style_tab_unselected->duplicate();
	style_theme_preview_bg_tab->set_expand_margin(SIDE_BOTTOM, 2 * EDSCALE);
	theme->set_stylebox("ThemeEditorPreviewBG", "EditorStyles", style_theme_preview_bg_tab);
	
	theme->set_stylebox("separator", "HSeparator", make_line_stylebox(separator_color, MAX(Math::round(EDSCALE), border_width)));
	theme->set_stylebox("separator", "VSeparator", make_line_stylebox(separator_color, MAX(Math::round(EDSCALE), border_width), 0, 0, true));
	
	Ref<StyleBoxFlat> style_panel_debugger = style_content_panel->duplicate();
	style_panel_debugger->set_border_width(SIDE_BOTTOM, 0);
	theme->set_stylebox("DebuggerPanel", "EditorStyles", style_panel_debugger);
	Ref<StyleBoxFlat> style_panel_invisible_top = style_content_panel->duplicate();
	int stylebox_offset = theme->get_font(SNAME("tab_selected"), SNAME("TabContainer"))->get_height(theme->get_font_size(SNAME("tab_selected"), SNAME("TabContainer"))) + theme->get_stylebox(SNAME("tab_selected"), SNAME("TabContainer"))->get_minimum_size().height + theme->get_stylebox(SNAME("panel"), SNAME("TabContainer"))->get_content_margin(SIDE_TOP);
	style_panel_invisible_top->set_expand_margin(SIDE_TOP, -stylebox_offset);