fix(fields): add default value to prevent SQL error

[stonebuzz]

Changes description

Prevent SQL error when user does not select value from dropdown

[2022-09-06 10:11:13] glpisqllog.ERROR: DBmysql::query() in /home/dev/GLPI/10.0-bugfixes/src/DBmysql.php line 370
  *** MySQL query error:
  SQL: INSERT INTO `glpi_plugin_fields_ticketblocfcs` (`plugin_fields_containers_id`, `nativetextfield`, `plugin_fields_customdropdownfielddropdowns_id`, `plugin_fields_plannificationfielddropdowns_id`, `items_id`) VALUES ('5', '', '1', '', '28')
  Error: Incorrect integer value: '' for column 'plugin_fields_plannificationfielddropdowns_id' at row 1
  Backtrace :
  src/DBmysql.php:1312                               
  src/CommonDBTM.php:716                             DBmysql->insert()
  src/CommonDBTM.php:1316                            CommonDBTM->addToDB()
  plugins/fields/inc/container.class.php:1070        CommonDBTM->add()
  plugins/fields/inc/container.class.php:1414        PluginFieldsContainer->updateFieldsValues()
  src/Plugin.php:1476                                PluginFieldsContainer::postItemAdd()
  src/CommonDBTM.php:1363                            Plugin::doHook()
  plugins/formcreator/inc/targetticket.class.php:943 CommonDBTM->add()
  plugins/formcreator/inc/formanswer.class.php:869   PluginFormcreatorTargetTicket->save()
  plugins/formcreator/inc/formanswer.class.php:1091  PluginFormcreatorFormAnswer->generateTarget()
  src/CommonDBTM.php:1317                            PluginFormcreatorFormAnswer->post_addItem()
  plugins/formcreator/ajax/formanswer.php:58         CommonDBTM->add()
  {"user":"2@Desktop","mem_usage":"0.272\", 8.89Mio)"} 

Checklist

Please check if your PR fulfills the following specifications:

• Tests for the changes have been added
• Docs have been added/updated

References

Closes #N/A