title: Sequelize mysql query where attribute is null
body: <p>I have an address and I want to see if it's in the db already, if not, create a new one. I know I can use <code>findOrCreate()</code> here, but let's make it easy and just check why I can't even find the existing address.</p>
  
  <pre><code>  var address = {
      name: req.body.name,
      address1: req.body.address1,
      address2: req.body.address2,
      zip: req.body.zip,
      city: req.body.city,
      country: req.body.country,
      user_id: req.body.user_id
    };
  
    Address.find({where: address}).then(function(result){
      console.log(result);
    }).catch(function(err){
      console.error(err);
    });
  </code></pre>
  
  <p>The generated query asks for <code>... AND user_id = NULL</code>which is wrong. It should ask <code>... AND user_id IS NULL</code>. How can I let sequelize do it right for me? Thanks.</p>