Fix PHP 8.1 exceptions rendering task 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.

In the other case, do not call json_decode() when passing null to it.

EXCEPTION: (RuntimeException) json_decode(): Passing null to parameter #1 ($json) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
  #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]

EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldSelect.php:76]

Closes T15683

Test Plan: After configuring a custom select field, access a task.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15683

Differential Revision: https://we.phorge.it/D25487