expo-apple-auth libdef: Make identityToken, authorizationCode non-optional

The Apple docs are frustratingly vague, but it doesn't seem to be
the case that "no credential" and "yes credential, but no
`identityToken`/`authorizationCode`" are separate auth failure modes
that we have to distinguish.

As I theorize in an Expo issue, these fields being marked optional
does correspond to the Swift version of an Apple doc [1], where
they're also marked optional.

We still don't know exactly why that Apple doc has them marked
optional, but corresponding values in a different protocol, the one
you use on other platforms (i.e., not the native iOS flow), *are*
marked optional for a particular reason [2]. In particular, they
might be missing if you said you didn't want them. But...here, there
doesn't seem to be a way to say you don't want them. So, they must
be there.

Also, in the JavaScript layer of `expo-apple-authentication`, an
error is thrown if `identityToken` or `authorizationCode` is
missing. (See `signInAsync` in `build/AppleAuthentication.js`.)

[1]: expo/expo#8446 (comment)
[2]: expo/expo#8446 (comment)

Author: chrisbobbe

flow-typed/expo-apple-authentication_vx.x.x.js

@@ -164,13 +164,13 @@ declare module 'expo-apple-authentication/build/AppleAuthentication.types' {
     /**
      * A JSON Web Token (JWT) that securely communicates information about the user to your app.
      */
-    identityToken: string | null,
+    identityToken: string,
 
     /**
      * A short-lived session token used by your app for proof of authorization when interacting with
      * the app's server counterpart. Unlike `user`, this is ephemeral and will change each session.
      */
-    authorizationCode: string | null,
+    authorizationCode: string,
     ...
   };
   /**