Node Js Express validator Check if field is numeric only if it is not empty

You need to set the checkFalsy option for optional to allow defined-but-empty values:

request.checkBody({ 
  'product_price': {
    optional: {
      options: { checkFalsy: true }
    },
    isDecimal: {
      errorMessage: 'The product price must be a decimal'
    }
  }
});

EDIT: the documentation has been moved since posting this answer, it can now be found here.