Support php7.4 nullable typed properties for JMS serializer.

[zviryatko]

In the case of php7.4 typed properties for JMS serializer there is no difference between nullable typed property and required one:

    /**
     * @Serializer\Type("integer")
     */
    private int $id;

    /**
     * @Serializer\Type("string")
     */
    private ?string $name;

Spec is generated for both properties as optional, but the $id must be required.

This PR adds support for marking fields as required automatically for php7.4 typed properties and for virtual properties based on their return type's nullability.

[zviryatko]

Oops, it seems I need to move it out to a separate controller to avoid conflicts with <7.4 tests.

[zviryatko]

It seems master branch is corrupted, so waiting for fix upstream, later I will rebase.

[shakaran]

@zviryatko could you rebase with last updates? Thanks

[zviryatko]

@shakaran finally did that 🙄