title: Null object in hasOne relationship strange behaviour
body: <p>I am trying to create a User <code>resource</code> in <code>Laravel</code> where the an User <code>object</code> has a nested Login <code>object</code>.</p>
  
  <p>My User <code>class</code> have these <code>attributes</code>:</p>
  
  <pre><code>protected $fillable = [
      'name', 'nickname', 'dob',
  ];
  
  public function login(){
      return $this-&gt;hasOne(Login::class, 'id');
  }
  </code></pre>
  
  <p>My Login <code>class</code> is like this:</p>
  
  <pre><code>protected $fillable = [
      'email', 'password',
  ];
  
  protected $hidden = [
      'password',
  ];
  
  public function user(){
      return $this-&gt;belongsTo(User::class, 'login');
  }   
  </code></pre>
  
  <p>This is my UserController index <code>method</code>:</p>
  
  <pre><code>public function index()
  {
      $usr= User::with('login')-&gt;get();
  
      return response()-&gt;json(['user' =&gt; $usr], 200);
  }
  </code></pre>
  
  <p>With an <code>object</code> that have a Login it works pretty well, however, with a User that doesnt have a Login, it returns a User that have the last Login inserted on the Login table.</p>
  
  <p>Why <code>Laravel</code> is doing this?</p>