### 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)

setting->set_initial_value("text_editor/theme/highlighting/caret_color", caret_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/caret_background_color", caret_background_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/text_selected_color", text_selected_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/selection_color", selection_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/brace_mismatch_color", brace_mismatch_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/current_line_color", current_line_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/line_length_guideline_color", line_length_guideline_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/word_highlighted_color", word_highlighted_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/number_color", number_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/function_color", function_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/member_variable_color", member_variable_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/mark_color", mark_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/bookmark_color", bookmark_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/breakpoint_color", breakpoint_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/executing_line_color", executing_line_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/code_folding_color", code_folding_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/search_result_color", search_result_color, true);
		setting->set_initial_value("text_editor/theme/highlighting/search_result_border_color", search_result_border_color, true);
	} else if (text_editor_color_theme == "Godot 2") {
		setting->load_text_editor_theme();
	}
	
	theme->set_font("font", "CodeEdit", theme->get_font(SNAME("source"), SNAME("EditorFonts")));
	theme->set_font_size("font_size", "CodeEdit", theme->get_font_size(SNAME("source_size"), SNAME("EditorFonts")));
	Ref<StyleBoxFlat> code_edit_stylebox = make_flat_stylebox(EDITOR_GET("text_editor/theme/highlighting/background_color"), widget_default_margin.x, widget_default_margin.y, widget_default_margin.x, widget_default_margin.y, corner_radius);
	theme->set_stylebox("normal", "CodeEdit", code_edit_stylebox);
	theme->set_stylebox("read_only", "CodeEdit", code_edit_stylebox);
	theme->set_stylebox("focus", "CodeEdit", Ref<StyleBoxEmpty>(memnew(StyleBoxEmpty)));
	theme->set_icon("tab", "CodeEdit", theme->get_icon(SNAME("GuiTab"), SNAME("EditorIcons")));
	theme->set_icon("space", "CodeEdit", theme->get_icon(SNAME("GuiSpace"), SNAME("EditorIcons")));