### 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("drop_mark_color", "TabContainer", tab_highlight);
	theme->set_color("drop_mark_color", "TabBar", tab_highlight);
	theme->set_icon("menu", "TabContainer", theme->get_icon(SNAME("GuiTabMenu"), SNAME("EditorIcons")));
	theme->set_icon("menu_highlight", "TabContainer", theme->get_icon(SNAME("GuiTabMenuHl"), SNAME("EditorIcons")));
	theme->set_icon("close", "TabBar", theme->get_icon(SNAME("GuiClose"), SNAME("EditorIcons")));
	theme->set_icon("increment", "TabContainer", theme->get_icon(SNAME("GuiScrollArrowRight"), SNAME("EditorIcons")));
	theme->set_icon("decrement", "TabContainer", theme->get_icon(SNAME("GuiScrollArrowLeft"), SNAME("EditorIcons")));
	theme->set_icon("increment", "TabBar", theme->get_icon(SNAME("GuiScrollArrowRight"), SNAME("EditorIcons")));
	theme->set_icon("decrement", "TabBar", theme->get_icon(SNAME("GuiScrollArrowLeft"), SNAME("EditorIcons")));
	theme->set_icon("increment_highlight", "TabBar", theme->get_icon(SNAME("GuiScrollArrowRightHl"), SNAME("EditorIcons")));
	theme->set_icon("decrement_highlight", "TabBar", theme->get_icon(SNAME("GuiScrollArrowLeftHl"), SNAME("EditorIcons")));
	theme->set_icon("increment_highlight", "TabContainer", theme->get_icon(SNAME("GuiScrollArrowRightHl"), SNAME("EditorIcons")));
	theme->set_icon("decrement_highlight", "TabContainer", theme->get_icon(SNAME("GuiScrollArrowLeftHl"), SNAME("EditorIcons")));
	theme->set_icon("drop_mark", "TabContainer", theme->get_icon(SNAME("GuiTabDropMark"), SNAME("EditorIcons")));
	theme->set_icon("drop_mark", "TabBar", theme->get_icon(SNAME("GuiTabDropMark"), SNAME("EditorIcons")));
	theme->set_constant("side_margin", "TabContainer", 0);
	theme->set_constant("h_separation", "TabBar", 4 * EDSCALE);
	
	Ref<StyleBoxFlat> style_content_panel = style_default->duplicate();
	style_content_panel->set_border_color(dark_color_3);
	style_content_panel->set_border_width_all(border_width);
	style_content_panel->set_border_width(Side::SIDE_TOP, 0);
	style_content_panel->set_corner_radius(CORNER_TOP_LEFT, 0);
	style_content_panel->set_corner_radius(CORNER_TOP_RIGHT, 0);
	
	style_content_panel->set_content_margin_individual(margin_size_extra * EDSCALE, (2 + margin_size_extra) * EDSCALE, margin_size_extra * EDSCALE, margin_size_extra * EDSCALE);
	theme->set_stylebox("panel", "TabContainer", style_content_panel);
	
	Ref<StyleBoxFlat> style_bottom_panel = style_content_panel->duplicate();
	style_bottom_panel->set_corner_radius_all(corner_radius * EDSCALE);