title: Sequelize Alter always set all data table foreign key to null
body: <h2>Bug Description</h2>
  <p>after running sequelize sync with options <code>alter: true</code>, all foreign keys in tables set to <code>null</code>.</p>
  <pre class="lang-js prettyprint-override"><code>await sequelize.sync({ alter: true }).then(async (result) =&gt; {
        await initDataUserGroup();
        await initDataUserAdmin();
        await initDataGateType();
  });
  </code></pre>
  <p>and here my one model which has foreign key :</p>
  <pre class="lang-js prettyprint-override"><code>const { DataTypes } = require(&quot;sequelize&quot;);
  const { sequelize } = require(&quot;../services/DatabaseService&quot;);
  
  const UserGroup = require(&quot;./UserGroup&quot;);
  const Biodata = require(&quot;./Biodata&quot;);
  
  const User = sequelize.define(&quot;user&quot;, {
    username: {
      type: DataTypes.STRING,
      allowNull: false,
      unique: true,
      validate: {
        notNull: {
          msg: &quot;Username must not be empty&quot;,
        },
        notEmpty: true,
      },
    },
    email: {
      type: DataTypes.STRING,
      allowNull: false,
      unique: true,
      validate: {
        notNull: true,
        notEmpty: true,
        isEmail: true,
      },
    },
    password: {
      type: DataTypes.STRING,
      allowNull: false,
      validate: {
        notNull: true,
        notEmpty: true,
      },
    },
  });
  
  User.UserGroup = User.belongsTo(UserGroup);
  User.Biodata = User.hasOne(Biodata, {
    foreignKey: &quot;userId&quot;,
    as: &quot;biodata&quot;,
  });
  
  module.exports = User;
  </code></pre>
  <h3>What do you expect to happen?</h3>
  <p>It should not set all foreign keys to null after running sync with options <code>{ alter: true }</code></p>
  <h3>What is actually happening?</h3>
  <p><code>userGroupId</code> in table <code>user</code> is null and <code>userId</code> in table <code>biodata</code> is also null.</p>
  <h3>Environment</h3>
  <ul>
  <li>Sequelize version: ^6.6.5</li>
  <li>Node.js version: 12.9.1</li>
  <li>Dialect: SQLite (SQLite3)</li>
  </ul>
  <p>From my issue : <a href="https://github.com/sequelize/sequelize/issues/13464" rel="nofollow noreferrer">https://github.com/sequelize/sequelize/issues/13464</a></p>
  <p>anyone can help me please?</p>