Null need set to NULL in array type

This would be a BC Break and cannot be done anymore. You can add your own array type that has this behavior.

It will not be a loss of backward compatibility.
Now if we have an empty array, that is stored serialized NULL, but the default value for NULL instead of N;
On the part of php will not be any differences - the inverse transform is the same, from the database, we combined the two into a single value, the more accurate in terms of DB

The field might be NOT NULL, that is what might break BC.

and if field is null?

 

/**
 * @var array $yandexReg
 *
 * @ORM\Column(name="yandexReg", type="array", nullable=true)
 */
private $yandexReg;

Well in this case it works obviously, however the Type API doesn't have access to that information. Thats why I meant you should build your own array type and override the internal implementation. This is explained in the Mapping Fields Document of ORM and in the Type Documentation of DBAL.