title: Finding null values with .whereIn
body: <p>I have a built a query with Knex as follows:</p>
  
  <pre><code>knex('discounts')
    .where((builder) =&gt; {
      builder
        .where('product_code', productCode)
        .where((builder1) =&gt; {
          builder1
            .andWhere('account_code', customer)
            .orWhere('account_code', null);
        });
    })
  .select('*');
  </code></pre>
  
  <p>Everything works fine, but I felt like the <code>.where</code> statement is too long so tried to use the <code>.whereIn</code> function, and that's when I realized it doesn't work:</p>
  
  <pre><code>knex('discounts')
    .where((builder) =&gt; {
      builder
        .where('product_code', productCode)
        .whereIn('account_code', [customer, null]);
    })
  .select('*');
  </code></pre>
  
  <p>I am aware that we cannot use <code>null</code> with an <code>IN</code> in raw SQL and it should be done like so:</p>
  
  <pre><code>WHERE 
  (columnName IN ('value1', 'value2', 'value3') OR columnName IS NULL)
  </code></pre>
  
  <p>My question: is my initial query the only way to achieve this or is there any alternative way to use <code>.whereIn</code> with a <code>null</code> when using Knex?</p>