This fixes two issues when editing users:

- Saving a user without a primary mentor was failing due to a bad cast between

  the empty string and ObjectId. I've fixed it so that the frontend will

  serialize an empty primary mentor to null instead.

- An empty groups field contained an empty <span class="label"> element,

  which was making it impossible to click to trigger the editor. I've added a

  check so that the element is truly empty, which x-editable will show as an

  italic "Empty", making it clickable.

Used both of these to set up our latest student.