Auth Emulator batchGet should return nextPageToken if page is full

[yuchenshi]

Expected Behavior

If there are enough users to fill a whole page size (whatever that is), a nextPageToken should be returned even if the next page would be empty. (I've tested in production and this is the behavior indeed.)

Actual Behavior

The Auth Emulator returns no page token if number of users is same as page size. This causes tests failures like https://github.com/firebase/firebase-admin-node/pull/1204/checks?check_run_id=2189478758.

We had to disable that tests for now (firebase/firebase-admin-node#1205) and that should be reverted after the fix land in a release.