Trailing semicolon in RRULE throws a warning

[tacman]

Description of the Issue:

ICS: https://amissvillevfr.org/ical.html?from=calendar&id=73874

if the rule has a trailing ;, list() throws a warning, e.g.

"FREQ=MONTHLY;INTERVAL=1;BYDAY=1WE;"

foreach (explode(';', $anEvent['RRULE']) as $s) {
    list($k, $v) = explode('=', $s);
}

I'll submit a PR associated with this issue.