support MS Exchange vcal files with reminder at time of event

[lotheac]

I created an event in MS Exchange that exported as the following vcal when sent through email or exported through OWA's web API:

BEGIN:VCALENDAR
METHOD:PUBLISH
PRODID:Microsoft Exchange Server 2010
VERSION:2.0
BEGIN:VTIMEZONE
TZID:FLE Standard Time
BEGIN:STANDARD
DTSTART:16010101T040000
TZOFFSETFROM:+0300
TZOFFSETTO:+0200
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:16010101T030000
TZOFFSETFROM:+0200
TZOFFSETTO:+0300
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
ORGANIZER;CN=Lauri Tirkkonen:MAILTO:lauri@example.com
SUMMARY;LANGUAGE=en-US:test event
DTSTART;TZID=FLE Standard Time:20190402T160000
DTEND;TZID=FLE Standard Time:20190402T163000
UID:040000008200E00074C5B7101A82E008000000000F3FBF812AE9D401000000000000000
 01000000018FD4114B2F2BD4FA7EB9FAE73B49A34
CLASS:PUBLIC
PRIORITY:5
DTSTAMP:20190402T080323Z
TRANSP:OPAQUE
STATUS:CONFIRMED
SEQUENCE:0
LOCATION;LANGUAGE=en-US:
X-MICROSOFT-CDO-APPT-SEQUENCE:0
X-MICROSOFT-CDO-OWNERAPPTID:2117342223
X-MICROSOFT-CDO-BUSYSTATUS:FREE
X-MICROSOFT-CDO-INTENDEDSTATUS:BUSY
X-MICROSOFT-CDO-ALLDAYEVENT:FALSE
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-CDO-INSTTYPE:0
X-MICROSOFT-DISALLOW-COUNTER:FALSE
BEGIN:VALARM
DESCRIPTION:REMINDER
TRIGGER;RELATED=START:P
ACTION:DISPLAY
END:VALARM
END:VEVENT
END:VCALENDAR

it has a peculiarity: TRIGGER;RELATED=START:P, since I set the reminder to be at start of the event. This contradicts with https://tools.ietf.org/html/rfc5545#section-3.3.6 but what can you do...

This PR causes this peculiar case to be handled like 'PT0S'.

While here I also noticed that when a , is read to start another duration string, week/day/hour/minute/sec values are reinitialized to None, but they are initialized to 0 above, not None - the other commit fixes that too.