Fatal error: Uncaught Exception: Invalid iCal date format with "DTEND:None" in data#340

### PHP Version 8.1 ### PHP date.timezone America/Chicago ### ICS Parser Version 3.4.0 ### Operating System Linux ### Description Recently I've heard from a few users that they're getting this error: Fatal error: Uncaught Exception: Invalid iCal date format. I've traced it back to one particular calendar vendor whose output includes the line `DTEND:None` when events do not have an end date/time set. It's my understanding/assumption that this is _not_ a valid `DTEND` value. But, nevertheless, it's showing up in feeds. ### Steps to Reproduce BEGIN:VCALENDAR PRODID:Redacted VERSION:2.0 BEGIN:VEVENT SUMMARY:Redacted DTSTART;TZID=Europe/Berlin:20230112T180000 DTEND:None DTSTAMP:20240627T162224Z UID:2e408c67c5c649bd3128 CLASSIFICATION:Redacted DESCRIPTION:Redacted LOCATION:Redacted URL:https://example.com END:VEVENT END:VCALENDAR