title: Node Js Express validator Check if field is numeric only if it is not empty
body: <p>I use express-validator to check my post fields, one of my field must be a decimal or empty. I used this Schema:</p>
  
  <pre><code>request.checkBody({ 
          'product_price': {
              optional: true,
              isDecimal: {
                  errorMessage: 'The product price must be a decimal'
              }
          }
  })
  </code></pre>
  
  <p>The problem with this Schema is to not validate my post if product_price is empty, even with "optional: true".</p>