Fix(Core): prevent php warning

[stonebuzz]

Using self assignment

raises up a php warning

   {"user":"2@stanislas-asus-desktop"} 
[2024-08-27 13:24:55] glpiphplog.WARNING:   *** PHP Warning (2): Undefined array key "id" in /home/DEV/GLPI/10.0-bugfixes/plugins/escalade/inc/ticket.class.php at line 610
  Backtrace :
  plugins/escalade/inc/ticket.class.php:674          PluginEscaladeTicket::removeAssignUsers()
  plugins/escalade/hook.php:482                      PluginEscaladeTicket::item_add_user()
  src/Plugin.php:1713                                plugin_escalade_item_add_user()
  src/CommonDBTM.php:1390                            Plugin::doHook()
  src/CommonITILObject.php:1987                      CommonDBTM->add()
  src/Ticket.php:1345                                CommonITILObject->prepareInputForUpdate()
  src/CommonDBTM.php:1613                            Ticket->prepareInputForUpdate()
  front/ticket.form.php:210                          CommonDBTM->update()
  public/index.php:82                                require()