What should PicardMetricsTask do when the prefix is None and the input file is stdin?

[nh13]

Currently it does this:

  /** Method that should be used to fetch the file prefix, instead of accessing `prefix` directly. */
  def pathPrefix: PathPrefix = prefix getOrElse PathUtil.removeExtension(in)

Should it instead pipe it to /dev/null? Throw an exception? Let it crash later?

I am logging this issue so I can think about it later.