Fix PHP 8.1 "strlen(null)" exceptions which block rendering page to import ICS calendar file

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.

Closes T15362

Test Plan: Applied these two changes and /calendar/import/edit/ correctly rendered the "Create Import" page in the web browser, with the expected error about the missing ICS file (instead of an exception stacktrace).

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15362

Differential Revision: https://we.phorge.it/D25200