Fix PHP 8.1 "strlen(null)" exception which blocks rendering errors on Create Blueprint page

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=master, ref.master=e4fd31ec024e), phorge(head=D25240, ref.master=b1edfea09bad, ref.D25240=b1edfea09bad)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/infrastructure/customfield/standard/PhabricatorStandardCustomField.php:392]
```
Closes T15413

Test Plan:
Applied this change on top of D25239. Afterwards, the "Create Blueprint" page on `/drydock/blueprint/edit/form/default/` after pressing the "Create Blueprint" button the page correctly renders the expected error that some fields cannot be empty.

# Enter a commit message.
#
# Changes:
#
#     src/infrastructure/customfield/standard/PhabricatorStandardCustomField.php

Also, try to create extra numeric fields (like in the User application) and try to send zero or 1 using Conduit `user.edit`.

Reviewers: O1 Blessed Committers, valerio.bozzolan, speck

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

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

Maniphest Tasks: T15413

Differential Revision: https://we.phorge.it/D25240

src/infrastructure/customfield/standard/PhabricatorStandardCustomField.php

@@ -389,7 +389,7 @@ abstract class PhabricatorStandardCustomField
     if (is_array($value)) {
       return empty($value);
     }
-    return !strlen($value);
+    return $value === null || !strlen($value);
   }
 
   public function getApplicationTransactionTitle(