fix snapshot creation logic and data callback return type

[vcooley]

Addresses #661

Note that this brings the plugin API in line with the web API which could cause breaking changes for those using checks like DocumentSnapshot.data === null. If you want to use checks like that, you should use DocumentSnapshot.data() === undefined now.

[EddyVerbruggen]

Awesome! I’ll review this shortly.

[vcooley]

I'm actually looking a little deeper. Looks like I'd need to update in multiple places in order to ensure that the data method returns undefined. What do you think about actually changing the DocumentSnapshot constructor to modify the data method if exists === false?

i.e. this:

export class DocumentSnapshot implements firestore.DocumentSnapshot {
  constructor(public id: string, public exists: boolean, public data: () => firestore.DocumentData) {
    if (!exists) {
     this.data = () => undefined; 
   }
  }
}

[EddyVerbruggen]

That's a great suggestion! The goal is to be as much Web-API-compatible as possible, so I love this change. I'll merge and add your suggestion as well. THANKS 👍