arrow

ARROW-17085: [R] group_vars() should not return NULL (#13621)

If an ungrouped data.frame or an `arrow_dplyr_query` is given to `dplyr::group_vars()`, `character()` returns.
But for an ungrouped Table, `NULL` is returned.

```r
mtcars |> dplyr::group_vars()
#> character(0)
mtcars |> arrow:::as_adq() |> dplyr::group_vars()
#> character(0)
mtcars |> arrow::arrow_table() |> dplyr::group_vars()
#> NULL
```

Therefore, functions that expect `group_vars` to return character, such as the following, will fail.

```r
mtcars |> arrow::arrow_table() |> dtplyr::lazy_dt()
#> Error in new_step(parent, vars = names(parent), groups = groups, locals = list(), : is.character(groups) is not TRUE
```

This PR modifies `dplyr::group_vars()` and `dplyr::groups()` for Arrow objects to work the same as for data.frame.
(Note that `arrow_dplyr_query` already works the same way as data.frame.)

Lead-authored-by: SHIMA Tatsuya <ts1s1andn@gmail.com>
Co-authored-by: eitsupi <50911393+eitsupi@users.noreply.github.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>