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 freebsd#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 <freebsd#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/calendar/import/PhabricatorCalendarImportEngine.php:459]
```

```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter freebsd#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 <freebsd#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

Author: aklapper

src/applications/calendar/import/PhabricatorCalendarImportEngine.php

@@ -447,7 +447,7 @@ private function getFullNodeUID(PhutilCalendarEventNode $node) {
   private function getParentNodeUID(PhutilCalendarEventNode $node) {
     $recurrence_id = $node->getRecurrenceID();
 
-    if (!strlen($recurrence_id)) {
+    if (!phutil_nonempty_string($recurrence_id)) {
       return null;
     }
 
@@ -456,7 +456,7 @@ private function getParentNodeUID(PhutilCalendarEventNode $node) {
 
   private function getNodeInstanceEpoch(PhutilCalendarEventNode $node) {
     $instance_iso = $node->getRecurrenceID();
-    if (strlen($instance_iso)) {
+    if (phutil_nonempty_string($instance_iso)) {
       $instance_datetime = PhutilCalendarAbsoluteDateTime::newFromISO8601(
         $instance_iso);
       $instance_epoch = $instance_datetime->getEpoch();