Fix PHP 8.1 "json_decode(null)" exception editing a form when custom field of type Users exists
Summary:
When `$value` is `null`, do not pass `$value` down the stack in `buildControl()` to ultimately end up with `json_decode` complaining. Instead, just skip this call.
```
EXCEPTION: (RuntimeException) json_decode(): Passing null to parameter #1 ($json) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=customOAuthUrlencodeNull, ref.master=788098096e11, ref.customOAuthUrlencodeNull=4f0f2043b7e9), phorge(head=customFieldDate, ref.master=bcfcd9acfc12, ref.customFieldDate=ae8cbe84252d)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/error/PhutilErrorHandler.php:261]
#1 <#2> json_decode(NULL, boolean) called at [<phorge>/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldPHIDs.php:44]
#2 <#2> PhabricatorStandardCustomFieldPHIDs::setValueFromStorage(NULL) called at [<phorge>/src/infrastructure/customfield/field/PhabricatorCustomField.php:895]
#3 <#2> PhabricatorCustomField::setValueFromApplicationTransactions(NULL) called at [<phorge>/src/infrastructure/customfield/editor/PhabricatorCustomFieldEditField.php:70]
#4 <#2> PhabricatorCustomFieldEditField::buildControl() called at [<phorge>/src/applications/transactions/editfield/PhabricatorEditField.php:385]
```
Closes T15602
Test Plan: After applying these three changes and creating a custom field with `"type": "users"` under `/config/edit/maniphest.custom-field-definitions/`, the website `/transactions/editengine/maniphest.task/view/5/` renders correctly in the browser, showing "This is a preview of the current form configuration."
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15602
Differential Revision: https://we.phorge.it/D25390