bug#56078: 29.0.50; url-handler-mode fails to open FTP link

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Turn on url-handler-mode, and then visit the URL

  ftp://ftp.cam.ov.com/pub/xrn/unsupported/xrn-motif.tgz

with find-file.  A (wrong-type-argument stringp nil) error will result.

In GNU Emacs 29.0.50 (build 107, x86_64-pc-linux-gnu)
 of 2022-06-19 built on trinity
Repository revision: ec6f6d23eb37ebd4fad17eb02730bf463aa27c5a
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Fedora Linux 36 (Workstation Edition)

Configured using:
 'configure --with-dumping=unexec --with-x-toolkit=no --without-cairo
 --cache-file=/tmp/ccache'

Configured features:
ACL DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON LCMS2
LIBSELINUX LIBSYSTEMD LIBXML2 MODULES NOTIFY INOTIFY OLDXMENU PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF UNEXEC WEBP X11 XDBE XFT XIM XINPUT2
XPM ZLIB

Important settings:
  value of $LANG: en_GB.UTF-8
  value of $XMODIFIERS: @im=none
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media rmc puny
dired dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg
rfc6068 epg-config gnus-util text-property-search time-date seq gv
subr-x byte-opt bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
iso-transl tooltip eldoc paren electric uniquify ediff-hook vc-hooks
lisp-float-type elisp-mode mwheel term/x-win x-win term/common-win x-dnd
tool-bar dnd fontset image regexp-opt fringe tabulated-list replace
newcomment text-mode lisp-mode prog-mode register page tab-bar menu-bar
rfn-eshadow isearch easymenu timer select scroll-bar mouse jit-lock
font-lock syntax font-core term/tty-colors frame minibuffer nadvice
simple cl-generic indonesian philippine cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite emoji-zwj charscript charprop case-table
epa-hook jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button
loaddefs faces cus-face macroexp files window text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting xinput2 x
multi-tty make-network-process emacs)

Memory information:
((conses 16 105814 7101)
 (symbols 48 21268 0)
 (strings 32 30631 1795)
 (string-bytes 1 821829)
 (vectors 16 17121)
 (vector-slots 8 554015 9039)
 (floats 8 59 68)
 (intervals 56 260 0)
 (buffers 992 10))