Can use vctrs::vec_proxy_order() now

use mask^$get_current_group() to get the current group, rather than ì` (#5559)

use mask^$get_current_group() to get the current group, rather than ì`

avoid INTEGER(x) when x is size 0. (#5558)

+unit test based on @earowang 's code

Update src/group_data.cpp