title: allow null value for sequelize foreignkey?
body: <p>How do I allow null for a foreignkey? I have a <code>through</code> and <code>belongsToMany</code> association:</p>
  
  <pre><code>Shelf
      belongsToMany(Book, { through: Library, as: "Books"});
  
  Book
      belongsToMany(Shelf, { through: Library, as: "Shelves"});
  
  Library
      section: DataTypes.STRING,
      // ... other fields
  </code></pre>
  
  <p>I would like to record a <code>null</code> on the <code>bookId</code> in <code>Library</code>:</p>
  
  <pre><code>Library.create({
      section: "blah",
      bookId: null,
      shelfId: 3
  });
  </code></pre>
  
  <p>Since Sequelize automatically creates the <code>bookId</code> and <code>shelfId</code> in the join table <code>Library</code>, how would I specify that I want to allow a <code>null</code> on the <code>bookId</code> foreignkey in <code>Library</code>? </p>