Fix error when no fields are updated

[cedric-anne]

Fixes following errors:

[2023-09-14 10:50:57] glpiphplog.WARNING: *** PHP Warning (2): Undefined variable $updates in C:\inetpub\wwwroot\marketplace\fields\inc\container.class.php at line 1330
Backtrace :
marketplace\fields\inc\container.class.php:1195 PluginFieldsContainer::constructHistory()
marketplace\fields\inc\container.class.php:1595 PluginFieldsContainer->updateFieldsValues()
src\Plugin.php:1475 PluginFieldsContainer::preItemUpdate()
src\CommonDBTM.php:1591 Plugin::doHook()
front\ticket.form.php:84 CommonDBTM->update()

[2023-09-14 10:50:57] glpiphplog.WARNING: *** PHP Warning (2): foreach() argument must be of type array|object, null given in C:\inetpub\wwwroot\marketplace\fields\inc\container.class.php at line 1330
Backtrace :
marketplace\fields\inc\container.class.php:1195 PluginFieldsContainer::constructHistory()
marketplace\fields\inc\container.class.php:1595 PluginFieldsContainer->updateFieldsValues()
src\Plugin.php:1475 PluginFieldsContainer::preItemUpdate()
src\CommonDBTM.php:1591 Plugin::doHook()
front\ticket.form.php:84 CommonDBTM->update()