Fix/table and ldap question empty selection sentinel

[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 !46011
• Here is a brief description of what this PR does

LdapQuestion and TableQuestion's Actor/Item(Dropdown) columns are rendered with GLPI's shared ajax dropdown widget, which always exposes a "-----" (empty) choice whose id is 0. Neither question type filtered that sentinel the way core's AbstractQuestionTypeActors already does for its own dropdown, so selecting the empty choice or clearing a previous selection saved and displayed a literal "0" instead of an empty answer.

Changes

• LdapQuestion::prepareEndUserAnswer(): treat "0" as no answer.
• TableQuestion: new normalizeDropdownCells() helper, applied in both prepareEndUserAnswer() and validateAnswer(), treats "0" as empty for Actor/Item(Dropdown) columns only (other column types, e.g. Number, keep 0 as a valid value).

Screenshots

Before :

After :