Fix PHP 8.1 "strlen(null)" exception mass-editing tasks when custom Date field present

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/PhabricatorStandardCustomFieldDate.php:113]

ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDate.php:120]

Closes T15794

Test Plan:
Define a date field in maniphest.custom-field-definitions, have more than 100 Maniphest tasks, and select some to bulk-edit them. Example config:

{
  "mycompany.deadline": {
    "name": "Deadline",
    "type": "date",
    "caption": "Estimated deadline.",
    "search": true
  }
}

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15794

Differential Revision: https://we.phorge.it/D25599