title: joi validation: Set minimum array length conditionally
body: <p>I have an array field which i would like to ensure that it has at least one element when a condition is met:</p>
  
  <pre><code>genre:Joi.array().includes(data.genres).when('field'{is:'fieldValue',then:Joi.required()})
  </code></pre>
  
  <p>If i changed the 'then' field with Joi.required().min(1), it complains. </p>
  
  <p>Can i do this with Joi?</p>