Gtk::UiTemplate always requires children

[BlobCodes]

When trying to assign a ui template to a class without any children, you can't just leave out the children field in the annotation.

@[Gtk::UiTemplate(file: "(some path)", children: [] of Symbol)]
class Test < Gtk::Box
end

This works.

@[Gtk::UiTemplate(file: "(some path)")]
class Test < Gtk::Box
end

This raises the following exception:

In lib/gtk4/lib/gtk4/src/bindings/gtk/widget_template.cr:45:58

 45 | {% for child in @type.annotation(Gtk::UiTemplate)[:children] %}
                                                       ^
Error: `for` expression must be an array, hash, tuple, named tuple or a range literal, not NilLiteral:

nil