Fix validation problem on internals.docs to allow swaggerUI to properly load the /docs url without a query present

I had an issue with swaggerUI not being able to load because it couldn't access the /docs url because of the validation on internals.docs in index.js requiring the query to be present.

To remedy it, I made the query optional by wrapping the query keys in Joi.object().keys()
i.e:
```
validate: {
            query: Joi.object().keys({
                path: Joi.string(),
                tags: Joi.string(),
                api_key: Joi.string()
            })
        },
...
```
I'm using Joi 4.6.2 and Hapi 6.6.0.