If a keyring is empty, store a no data flag instead of failing.

Any fully empty keyring (no keys and no applications) would fail to add
the empty flat buffer and thus prevent the whole keystore database from
being stored. This could easily happen when you used separate keyrings
but the master keyring was left unused for example.

Adding a flag that tells that there is no data allows us to distinguish
between a case where the stored data is missing due to a problem versus
an actually empty buffer.