How to check existence of an input argument for R functions without using '...'

Question

Please consider the following example to check for my arguments:

fun <- function(x, y, z) { 
  notexist <- is.null(y)
  return(notexist)
}

fun(x) # Should return TRUE since y is missing
fun(x,y) # Should return FALSE since y is being passed

I have come across a similar post which makes use of three-dots construct How to check existence of an input argument for R functions. But I do not want to use exists, missing, get, hasArg and three-dots construct. But instead I want to write this code by just using the args and is.null()? Is it possible without three-dots?