Flexible sync type definitions require strictNullChecks to be enabled

[tomduncalf]

The type definitions for flexible sync require strictNullChecks to be enabled in the user's tsconfig.json otherwise they will get errors: https://github.com/realm/realm-js/blob/master/types/index.d.ts#L144-L145

I can't remember the details of why this is so, but Kræn and I did quite a bit of investigation and concluded that the type discrimination using the flexible: true and flexible?: false | undefined did not work properly without strict null checks. I think there was a somewhat logical explanation for this, I can try to cast my mind back if useful!

We thought this would be a non-issue in practice for the majority of users, but unfortunately it turns out that Expo defaults to strict mode disabled so there is a chance that Expo users will hit this issue. I'm not sure if there is a good workaround, but it warrants further investigation.