Function opt_str_iter_to_strsxp 

pub(crate) fn opt_str_iter_to_strsxp<'a>(
    iter: impl ExactSizeIterator<Item = Option<&'a str>>,
) -> SEXP

Helper: allocate STRSXP and fill from an optional-string iterator (checked).

None becomes NA_character_. Shared by the Vec<Option<…str>> impls.