`null` is a valid default option for the `ttl` of the cookie but the schema isn't currently allowing it.

As an example, in our case the value for `ttl` is coming from an external place; from a configuration system which sets it on a per-env basis.
In test/development the `ttl` might be set to an explicit value whereas in production it might be `null` 

```js
        server.auth.strategy('session', 'cookie', 'try', {
            password: options.cookieOptions.password,
            cookie: 'ssosid',
            ttl: options.cookieOptions.ttl,
            domain: options.cookieOptions.domain,
            redirectOnTry: false,
            appendNext: true,
            clearInvalid: true,
            isSecure: true,
            isHttpOnly: true,
            validateFunc: validate
        });
```

We can work around this easily by doing something like:
```js
        const strategyOptions = {
            password: options.cookieOptions.password,
            cookie: 'ssosid',
            domain: options.cookieOptions.domain,
            redirectOnTry: false,
            appendNext: true,
            clearInvalid: true,
            isSecure: true,
            isHttpOnly: true,
            validateFunc: validate
        };

        if(options.cookieOptions.ttl) {
            strategyOptions.ttl = options.cookieOptions.ttl;
        }

        server.auth.strategy('session', 'cookie', 'try', strategyOptions);
```

However, it just felt a little cleaner to have the `ttl` schema accept a valid cookie ttl value.

Thoughts?