Re: (substitute-in-file-name "$NONEXISTENT") shouldn't throw error

[Christoph Conrad]

In GNU Emacs 21.1.1 (i586-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2001-10-22 on mutzel
configured using `configure  --with-x'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: german
  locale-coding-system: nil
  default-enable-multibyte-characters: nil

see subject. It should be configurable if an error is thrown (default)
or the nonexistent variable is expanded to a nullstring.

Rationale: The bash expands nonexistent variables to a nullstring.