Fix PHP 8.1 "strlen(null)" exception when custom select field configured

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.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/infrastructure/customfield/standard/PhabricatorStandardCustomField.php:484]
```

Closes T15687

Test Plan:
Unknown. Definitely requires having custom fields defined, then playing with creating tasks using forms which expose these fields and going to `/maniphest/query/all/`.
See also D25487.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15687

Differential Revision: https://we.phorge.it/D25492

src/infrastructure/customfield/standard/PhabricatorStandardCustomField.php

@@ -481,7 +481,7 @@ abstract class PhabricatorStandardCustomField
     }
 
     $field_value = $this->getFieldValue();
-    if (strlen($field_value)) {
+    if (($field_value !== null) && (strlen($field_value))) {
       $document->addField($field_key, $field_value);
     }
   }