Arvados

In `app/controllers/arvados/v1/groups_controller.rb` we say

  def self._contents_requires_parameters
    params = _index_requires_parameters.
      merge({
              uuid: {
                type: 'string', required: false, default: nil,
              },
...

That `default: nil` ends up in our discovery doc as

            "uuid": {
              "type": "string",
              "required": false,
              "default": null,
              "description": "",
              "location": "query" 
            },

That is the only place in our discovery doc where we say `"default": null`, and that does not conform with the spec. The value for `"default"` must be a string, as per https://developers.google.com/discovery/v1/reference/apis.