title: Mongoose array required in schema validation not working
body: <p>In my schema I need to have a propery that is an array that must be always not null and not undefined.</p>
  
  <p>So I defined it required, but the validation is not working as I expected, because if I omit the property no error is throw.</p>
  
  <p>In case of simple property (not an array) this work as I expected</p>
  
  <pre><code>const nodeSchema = new Schema({
      address: { type: String, required: true },
      outputs: { type: [String], required: true }
  })
  </code></pre>