Replace use of is.null() by missing()

[egonw]

Orginally reported by Maarten-Jan Kallen on rOpenSci for one method (that one actually is applied, but there is more such use):

No, you should just do

add.prefix <- function(store, prefix, namespace) {
      if (missing(store)) stop("A store must be given.")
      if (missing(prefix)) stop("A prefix must be given.")
      if (missing(namespace)) stop("A namespace must be given.")
# ...
}

Unfortunately, the original bug report is lost.