Remove dependency on ellipsis.

[olivroy]

Replace ellipsis::check_dots_empty() by rlang::check_dots_empty()

Since there is #' @import rlang, removing ns suffices, as rlang has check_dots_empty()

[gadenbuie]

Thanks @olivroy!

And a note that check_dots_empty() was added in rlang 1.0.0. htmltools already depends on rlang >= 1.0.0 so this change is compatible with htmltools' dependency list.

[cpsievert]

Thanks!