Fix nullable detection

[DrWarpMan]

In Symfony production environment, since Doctrine 3.x, cached values for 'nullable' property can be either null, false or true. In Doctrine 2.x, it's only false or true. When Doctrine returns null for the nullable property, the required option for the field is set to false.

While in fact, Doctrine considers nullable => null as nullable => false, thus the field should be required.

To reproduce:

• create an entity, with a field that has 'nullable' property set to false
• create an admin controller for the entity (make sure to NOT set setRequired(true) explicitly)
• try to create a new record in the dashboard, you will see that the field is optional

References:
Doctrine 3.x $nullable
How Doctrine determines nullable

[ServerExe]

This is some kind of strange behavior of Doctrine or EasyAdmin. But changing the APP_ENV to prod and removing the var/cache folder leads to exactly this behavior as it is on production. When debugging, it's really hard to find the turning point in the code where it takes another path of workflow. I am not sure but somehow it might have something to do with Doctrine Caching system. Because on production mode it doesn't even call the AdminContextFactory, where actually some metadata would be loaded from the entities. I am really not sure if my information is valid but as it seems production has a completely different behavior. I realized that when loading the PHP attributes of the entity via Reflection it recognizes the #[ORM\Id] and #[ORM\Column] attributes on DEV mode but it completely ignores them on production mode. So the default KeyValueStore of these attributes aren't even loaded ...

I can't rely on the entity property detection anymore and will add the required flag to all of my fields in the crud controllers :(