sync/v4 expects to return an empty root instead of 404

[nemunaire]

In this patch, I tried do fix issues reported on the initial synchronization, when the account is new and no sync happen before.

I was able to test the creation of a new account with the sync/v3, it works as expected, but the changes between v3 and v4 seems to be related to how the function behaves for new accounts: https://github.com/ddvk/rmfakecloud/issues/311#issuecomment-2271359544 https://github.com/ddvk/rmfakecloud/issues/326 https://github.com/ddvk/rmfakecloud/issues/270#issuecomment-2351103545