empty dots in `group_by()`, `mutate()`, and `transmute()` don't work correctly

[mgirlich]

library(dplyr, warn.conflicts = FALSE)
library(dtplyr)

dt <- lazy_dt(tibble(x = 1), "DT")
dt %>% group_by(x) %>% group_by()
#> Error in new_step(parent, vars = parent$vars, groups = groups, class = "dtplyr_step_group", : is.character(groups) is not TRUE
dt %>% as_tibble() %>% group_by(x) %>% group_by()
#> # A tibble: 1 x 1
#>       x
#>   <dbl>
#> 1     1

dt %>% mutate(!!!list())
#> Error in `[.data.table`(copy(DT), , `:=`()): In `:=`(col1=val1, col2=val2, ...) form, all arguments must be named.

dt %>% transmute()
#> Error in new_step(parent = parent, vars = vars, groups = groups, locals = locals, : is.character(vars) is not TRUE
dt %>% transmute(!!!list())
#> Error in new_step(parent = parent, vars = vars, groups = groups, locals = locals, : is.character(vars) is not TRUE

^{Created on 2021-05-19 by the reprex package (v2.0.0)}