title: Bidirectional @OneToOne not work when persisting child without parent
body: <p>I have this situation :</p>
  
  <p>Parent entity:</p>
  
  <pre><code>@Entity
  public class Address {
  
    @OneToOne( optional = false, targetEntity = User.class, fetch = FetchType.LAZY, orphanRemoval = true )
    @JoinColumn( name = "id_user", referencedColumnName = "id" )
    private User user;
  
  }
  </code></pre>
  
  <p>Child entity:</p>
  
  <pre><code>@Entity
  public class User {
  
     @OneToOne( optional = false, targetEntity = Address.class, mappedBy = "user", fetch = FetchType.LAZY, orphanRemoval = true )
     private Address address;
  }
  </code></pre>
  
  <p>As you can see there isn't any cascade operation in both sides. But when I save a user : </p>
  
  <pre><code>userRepository.save( new User() ); 
  </code></pre>
  
  <p>It throws this exception :</p>
  
  <pre><code>org.springframework.dao.DataIntegrityViolationException: not-null property references a null or transient value : org.company.models.User.address; nested exception is org.hibernate.PropertyValueException: not-null property references a null or transient value : org.company.models.User.address
  .............
  Caused by: org.hibernate.PropertyValueException: not-null property references a null or transient value : org.company.models.User.address
  </code></pre>
  
  <p>I want to persist only the child, it should be possible since the <code>@JoinColumn</code> is in the parent, so I don't need to set it before saving.<br>
  Could someone clarify me this issue? 
  Thanks a lot</p>