Fix PHP 8.1 "strlen(null)" exceptions importing ICS file into calendar

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.

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.

EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=df6c315ace5f), phorge(head=importICSCalendar, ref.master=3cc5ee6a33df, ref.importICSCalendar=3bd396120123)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/calendar/import/PhabricatorCalendarImportEngine.php:459]

EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=df6c315ace5f), phorge(head=importICSCalendar, ref.master=3cc5ee6a33df, ref.importICSCalendar=3bd396120123)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/calendar/import/PhabricatorCalendarImportEngine.php:450]

Closes T15620

Test Plan:

• Revert rP02a4f8b0c8f1279fc0040ad8077942fd8b0d948b not to run into T15619
• Try to import an ICS file via /calendar/import/edit/
• See that page /calendar/import/4/ renders correctly in web browser and shows Log Messages and Imported Events as expected

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15620

Differential Revision: https://we.phorge.it/D25411