Stronger Type Safety for Auth0Id

While creating PR #184 a concern arose wherein the type applied to the `auth0Id` value in a few places was a `string` which also happens to accept `null` and `undefined` as valid values.

The suggestion at the time was to manually coalesce falsey values to `''` using something like this:

```
auth0Id || ''
```

Which this will certainly work, I don't believe it's not an ideal habit for us to establish. More specifically, I don't think it's a good use of time to manually enforce a level of type safety that the type checker itself is allowing.

What I propose instead is one or both of the following options:

1. Enable strict null checks so that none of the types in the system allow null/undefined unless told to do so explicitly. More can be read on this [here](Nullable Types: https://www.typescriptlang.org/docs/handbook/advanced-types.html#nullable-types).
2. Make a custom type just for the `auth0Id` that adds type-safe validation as well. Here's [an article covering one potential approach](https://medium.com/@dhruvrajvanshi/advanced-typescript-patterns-6cf8826c7944).

I'm not expecting any immediate progress on this topic. I think it's wise for us to consider whether or not we want to solve this problem at all, and, if so, how soon we want to solve this problem.