Error about arguments of length zero when judge a dataframe is Null in R

Hi there I am new to R and I am trying to see if the df is Null, if it is, then assigned "empty" to it but there is an error like this, do you know how to solve it?

if(is.na(df)){
  df <- "Empty"}

Error in if (is.na(data_si)) { : argument is of length zero