How to save result as "ND" when there is no record? rvest and R

We could check the length and if it is 0 (length(character(0)) is 0), change the value to 'ND'

for (i in seq_len(nrow(x))){
    html<-x$page[i]%>% 
                 unclass() %>%
                 unlist()
    read_html(html,encoding = "ISO-8859-1") %>% 
        rvest::html_elements(xpath = '//*[@id="principal"]/table[2]') %>% 
        rvest::html_elements(xpath = '//div[@id="tituloContext"]') %>% 
        html_text()%>%  
        str_replace_all(.,"[\\n\\r\\t]+", "")%>%
        stringr::str_trim( ) -> tmp
      if(length(tmp) == 0) tmp <- "ND"
      x$title[i] <- tmp
}

-checking

> x$title
[1] "CARRINHO DE LIXO PARA LIMPEZA URBANA" "ND"