What correct way to return `NA_character`? ```r > cpp_eval('cpp11::r_string(NA_STRING)') [1] "NA" > cpp_eval('cpp11::writable::strings(NA_STRING)') [1] NA > cpp_eval('cpp11::writable::strings({NA_STRING})') [1] "NA" > cpp_eval('cpp11::writable::strings({NA_STRING, "text"})') [1] "NA" "text" ```