purge <- function(x,.p){

  x <- x[!sapply(x,.p)]

  if(inherits(x,'list')){
    lapply(x,purge,.p = .p)
  }else{
    x
  }

}

x <- list(NULL,list(a=list(1,NULL)),list(b=list(list(3,NA),2)))

purge(x,.p = is.null)

purge(x,.p=function(y) is.null(y)||is.na(y))