bug#43038: 28.0.50; gnus-icalendar-event:recurring-days fails when no BYDAY

[Filipp Gunbin]

Steps to repeat:

- Insert this in some buffer:

--8<---------------cut here---------------start------------->8---
BEGIN:VCALENDAR
METHOD:REQUEST
PRODID:Microsoft Exchange Server 2010
VERSION:2.0
BEGIN:VTIMEZONE
TZID:Russian Standard Time
BEGIN:STANDARD
DTSTART:16010101T000000
TZOFFSETFROM:+0300
TZOFFSETTO:+0300
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:16010101T000000
TZOFFSETFROM:+0300
TZOFFSETTO:+0300
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
ORGANIZER;CN=LastName FirstName:mailto:somebody@example.org
ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN=All_Company:m
 ailto:all@example.org
DESCRIPTION;LANGUAGE=en-US:\n
UID:040000008200E00074C5B7101A82E00800000000119F3EF5DC7AD601000000000000000
 0100000003B12F7DC2B09AC4383B5019AA7F618D8
SUMMARY;LANGUAGE=en-US:Title
DTSTART;TZID=Russian Standard Time:20200827T180000
DTEND;TZID=Russian Standard Time:20200827T203000
CLASS:PUBLIC
PRIORITY:5
DTSTAMP:20200825T124048Z
TRANSP:OPAQUE
STATUS:CONFIRMED
SEQUENCE:0
LOCATION;LANGUAGE=en-US:Townhall
BEGIN:VALARM
DESCRIPTION:REMINDER
TRIGGER;RELATED=START:-PT15M
ACTION:DISPLAY
END:VALARM
END:VEVENT
END:VCALENDAR
--8<---------------cut here---------------end--------------->8---

2) Eval in that buffer:

(gnus-icalendar-event:org-timestamp (gnus-icalendar-event-from-buffer
(current-buffer)))

Backtrace will start with this:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  string-match("BYDAY=\\([^;]+\\)" nil)


The problem started showing up after commit:

commit 5c715113efe91f4feb03e37b8b1287cd5afe9e94
Author: Jan Tatarik <jan.tatarik@gmail.com>
Date:   Sat Aug 22 15:39:17 2020 +0200

    gnus-icalendar does not understand multiple repeating days


Thanks.

In GNU Emacs 28.0.50 (build 1, x86_64-apple-darwin19.6.0, NS appkit-1894.60 
Version 10.15.6 (Build 19G73))
 of 2020-08-25 built on fgunbin.local
Repository revision: 787b19115378909c472f018629da763407f6e2a0
Repository branch: master
System Description:  Mac OS X 10.15.6