title: npm joi with null, undefined, empty values and a default value
body: <p>I am using older version of npm module joi => 10.2.2 and i am trying to figure out how can i build the schema so empty, null, undefined values are allowed with a default value. </p>
  
  <p>This works <a href="https://github.com/hapijs/joi/issues/516" rel="nofollow noreferrer">https://github.com/hapijs/joi/issues/516</a></p>
  
  <pre><code>var schema = joi.object().keys({a:[joi.string().optional(), joi.allow(null)]})
  </code></pre>
  
  <p>but i don't know how to specify a default value with this.</p>
  
  <p>What i am looking for is this <a href="https://github.com/hapijs/joi/issues/1066" rel="nofollow noreferrer">https://github.com/hapijs/joi/issues/1066</a> (version v10.5.2) but with the syntax of older version of joi. </p>