Empty TZID

One of our customer export ICS from "Microsoft Corporation//Outlook 14.0 MIMEDIR//EN"
Configured TZID is empty..

```
BEGIN:VTIMEZONE
TZID:
BEGIN:STANDARD
```

This commit allow a fallback to the default timezone value instead of a PHP fatal error.

lib/TimeZoneUtil.php

@@ -135,7 +135,7 @@ static function getTimeZone($tzid, Component $vcalendar = null, $failIfUncertain
         // Since PHP 5.5.10, the first bit will be used as the timezone and
         // this method will return just GMT+01:00. This is wrong, because it
         // doesn't take DST into account.
-        if ($tzid[0] !== '(') {
+        if (!empty($tzid) && $tzid[0] !== '(') {
 
             // PHP has a bug that logs PHP warnings even it shouldn't:
             // https://bugs.php.net/bug.php?id=67881