bug#46589: 28.0.50; Void variable regression in interactive spec

[Basil L. Contovounesios]

0. emacs -Q
1. M-x toggle-debug-on-error RET
2. M-x add-file-local-variable-prop-line RET

Debugger entered--Lisp error: (void-variable variable)
  (list variable (read-file-local-variable-value variable) t)
  call-interactively(add-file-local-variable-prop-line record nil)
  command-execute(add-file-local-variable-prop-line record)
  execute-extended-command(nil "add-file-local-variable-prop-line" 
"add-file-local-variable-")
  funcall-interactively(execute-extended-command nil 
"add-file-local-variable-prop-line" "add-file-local-variable-")
  call-interactively(execute-extended-command nil nil)
  command-execute(execute-extended-command)

This started with the following commit according to 'git bisect':

Extend the syntax of `interactive' to list applicable modes
58e0c8ee86 2021-02-14 13:21:24 +0100
https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=58e0c8ee86e2c36245f1c5a1483f1c73600b4914

-- 
Basil

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 
1.16.0, Xaw3d scroll bars)
 of 2021-02-17 built on tia
Repository revision: cccd701ac952f81da8444576a72d92b37ddf42d2
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Debian GNU/Linux bullseye/sid

Configured using:
 'configure 'CC=ccache gcc' 'CFLAGS=-O2 -march=native' --config-cache
 --prefix=/home/blc/.local --enable-checking=structs
 --with-x-toolkit=lucid --with-file-notification=yes --with-x'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11
XAW3D XDBE XIM XPM LUCID ZLIB

Important settings:
  value of $LANG: en_IE.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix