Fix PHP 8.1 exceptions importing ICS file without attendee names

Summary:
strlen() was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts phutil_nonempty_string() as a replacement.

Passing null to preg_match() is deprecated behavior since PHP 8.1.
Thus only call preg_match() when the value is set.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

ERROR 8192: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/calendar/import/PhabricatorCalendarImportEngine.php:238]

ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/calendar/import/PhabricatorCalendarImportEngine.php:245]

Closes T15852

Test Plan: Import an ICS file which lists attendees without a name but only with an email address into the calendar via http://phorge.localhost/calendar/import/ > "Import Events" > "Import .ics File"

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15852

Differential Revision: https://we.phorge.it/D25686