e-cal-util: Fix timezone clamp with DUE date only
Steps to reproduce:
- Create a new task
- set an expiration date and time
- save it to the local task list
- right click and save it to an ics file
BEGIN:VCALENDAR
PRODID:-//Ximian//NONSGML Evolution Calendar//EN
VERSION:2.0
METHOD:PUBLISH
BEGIN:VTIMEZONE
TZID:/freeassociation.sourceforge.net/Europe/Paris
X-LIC-LOCATION:Europe/Paris
END:VTIMEZONE
BEGIN:VTODO
UID:f0021b0b5ca1397caa92e5b56360941906949c25
DTSTAMP:20211228T212831Z
SUMMARY:test
DUE;TZID=/freeassociation.sourceforge.net/Europe/Paris:20220101T100000
PERCENT-COMPLETE:0
CLASS:PUBLIC
SEQUENCE:1
CREATED:20211231T130337Z
LAST-MODIFIED:20211231T130337Z
END:VTODO
END:VCALENDAR
This is invalid caldav format as the VTIMEZONE component does not contain at least one of standardc or daylightc: https://datatracker.ietf.org/doc/html/rfc2445#section-4.6.5
This is problematic with radicale: https://github.com/Kozea/Radicale/issues/1201
Evolution 3.42.1 NixOS 21.11 Linux x86_64
Edited by Milan Crha