Feat: update docData to instead return `Observable<T | null>`

_Related, currently, the `docData` helper exported by this library is incorrectly typed (see issue #37) as it actually returns `Observable`._

This is a request to update `docData` to return `Observable`. Returning `null` rather than `undefined` when a document doesn't exist would be helpful when saving the output to a variable as it is easier to distinguish between a pending result (`undefined`) and a known `null` result. I realize that this would be a breaking change.

Example:

```ts
let myDocument: { one: number } | null | undefined;

docData(/* args */).subscribe(doc => {
myDocument = doc;
})
```