R1:10cb2521630a

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…