Patch current_key() to work with zero row grouped data frames

[DavisVaughan]

Closes #6304

[DavisVaughan]

get_current_group() always returns a scalar integer.

It's a little tricky because private$keys is a zero row tibble when the original data frame is a zero row grouped-df. But we always evaluate at least 1 group in mutate(), even when the data frame is empty, so it looks like we are on group 1 when we call get_current_group(). Being on group 1 is right for when get_current_group() is used in cur_group_id() or current_rows(), but it isn't right here.

In general there are some tough edge cases with zero row grouped data frames.