default query string options do not work for empty query string
OS:  Max OS X 10.12.2
Node version: 8.6.0
npm version: 5.2.0
koa version: 2.4.1
koa-joi-router version: 5.0.0
joi version: 9.2.0

I have the following route:
```js
module.exports = {
  method: 'get',
  path: '/',
  validate: {
    query: Joi.object().keys({
      search: Joi.string(),
      sort: Joi.object().default({ 'name': 1 }),
    }),
  },
  handler: async (ctx) => {
    const { sort } = ctx.query;
    console.log(sort);
    ctx.status = 200;
  },
};
```

The actual behavior:
when I specify a query string ex. `?search=foo`, I get the correct default value for `sort` parameter, however for no query string I get `undefined`

The expected behavior:
even for no query string, the `sort` parameter should have `{ 'name': 1 }` as default value