Fix PHP 8.1 "strlen(null)" exception rendering Task with empty custom date field

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 #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=customOAuthUrlencodeNull, ref.master=df6c315ace5f, ref.customOAuthUrlencodeNull=c69b9749027f), phorge(head=master, ref.master=7868ab3754fa)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDate.php:14]
```

Closes T15632

Test Plan: Have a custom field with type=date defined in maniphest.custom-field-definitions and edit an existing Task changing something else, keeping that field as empty.

Reviewers: O1 Blessed Committers, Sten, valerio.bozzolan

Reviewed By: O1 Blessed Committers, Sten, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15632

Differential Revision: https://we.phorge.it/D25431