Fix - Convert empty dropdown value (-1) to null on form submission

[RomainLvr]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !43956
• Here is a brief description of what this PR does

When a form containing a Fields dropdown question (either via PluginFieldsQuestionType or the native QuestionTypeItemDropdown) is submitted without a value selected, GLPI sends -1 (or 0) as the items_id. This value was passed as-is to the INSERT query, but the underlying column is INT UNSIGNED, causing a MySQL "Out of range value" error and preventing ticket creation.

[2026-05-12 08:04:50] glpi.ERROR:   *** An error occured during the form `Test` submission: MySQL query error: Out of range value for column 'plugin_fields_dropdownfielddropdowns_id' at row 1 (1264) in SQL query "INSERT INTO `glpi_plugin_fields_tickettestblocs` (`plugin_fields_containers_id`, `plugin_fields_dropdownfielddropdowns_id`, `itemtype`, `items_id`, `entities_id`) VALUES ('2', '-1', 'Ticket', '8', '0')".
  Backtrace :
  ./src/DBmysql.php:416                              
  ./src/DBmysql.php:1377                             DBmysql->doQuery()
  ./src/CommonDBTM.php:778                           DBmysql->insert()
  ./src/CommonDBTM.php:1382                          CommonDBTM->addToDB()
  ./plugins/fields/inc/container.class.php:1430      CommonDBTM->add()
  ./plugins/fields/inc/container.class.php:1839      PluginFieldsContainer->updateFieldsValues()
  ./src/Plugin.php:1810                              PluginFieldsContainer::postItemAdd()
  ./src/CommonDBTM.php:1432                          Plugin::doHook()
  ...ation/AbstractCommonITILFormDestination.php:205 CommonDBTM->add()
  ./src/Session.php:2457                             Glpi\Form\Destination\AbstractCommonITILFormDestination->{closure:Glpi\Form\Destination\AbstractCommonITILFormDestination::createDestinationItems():205}()
  ...ation/AbstractCommonITILFormDestination.php:205 Session::callAsSystem()
  ...Glpi/Form/AnswersHandler/AnswersHandler.php:435 Glpi\Form\Destination\AbstractCommonITILFormDestination->createDestinationItems()
  ...Glpi/Form/AnswersHandler/AnswersHandler.php:288 Glpi\Form\AnswersHandler\AnswersHandler->createDestinations()
  ...Glpi/Form/AnswersHandler/AnswersHandler.php:206 Glpi\Form\AnswersHandler\AnswersHandler->doSaveAnswers()
  .../Controller/Form/SubmitAnswerController.php:156 Glpi\Form\AnswersHandler\AnswersHandler->saveAnswers()
  ...i/Controller/Form/SubmitAnswerController.php:83 Glpi\Controller\Form\SubmitAnswerController->saveSubmittedAnswers()
  ./vendor/symfony/http-kernel/HttpKernel.php:181    Glpi\Controller\Form\SubmitAnswerController->__invoke()
  ./vendor/symfony/http-kernel/HttpKernel.php:76     Symfony\Component\HttpKernel\HttpKernel->handleRaw()
  ./vendor/symfony/http-kernel/Kernel.php:208        Symfony\Component\HttpKernel\HttpKernel->handle()
  ./public/index.php:71                              Symfony\Component\HttpKernel\Kernel->handle()

Fix: In applyConfiguratedValueToInputUsingAnswers(), any items_id ≤ 0 is now converted to null before being added to the input, consistent with how GLPI core handles this (see EntityFieldStrategy).