Only nullify persisted has_one target associations

Since after 87d1aba3c `dependent: :destroy` callbacks on has_one
assocations run *after* destroy, it is possible that a nullification is
attempted on an already destroyed target:

    class Car < ActiveRecord::Base
      has_one :engine, dependent: :nullify
    end

    class Engine < ActiveRecord::Base
      belongs_to :car, dependent: :destroy
    end

    > car = Car.create!
    > engine = Engine.create!(car: car)
    > engine.destroy! # => ActiveRecord::ActiveRecordError: cannot update a
    >   destroyed record

In the above case, `engine.destroy!` deletes `engine` and *then* triggers the
deletion of `car`, which in turn triggers a nullification of `engine.car_id`.
However, `engine` is already destroyed at that point.

Fixes #21223.

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