Geometry Nodes: output Index from ID node if the geometry has no id

This is consistent with all the other places where we use the id attribute:
If it does not exist, use the index instead.

source/blender/nodes/geometry/nodes/node_geo_input_id.cc

@@ -25,7 +25,7 @@ static void geo_node_input_id_declare(NodeDeclarationBuilder &b)
 
 static void geo_node_input_id_exec(GeoNodeExecParams params)
 {
-  Field<int> position_field{AttributeFieldInput::Create<int>("id")};
+  Field<int> position_field{std::make_shared<bke::IDAttributeFieldInput>()};
   params.set_output("ID", std::move(position_field));
 }