AdminControlled: don't allow zero-address initialization

[sept-en]

In PR#705, new functionality to nominate and accept admin was added. One of that parts was verifying that a zero address value for admin is not allowed for security measures. However, by default, the constructor allows initialization with zero address admin. This should be changed and disallowed. In case it's the expected behavior to have no admin, the deployer should use some other address (e.g., with all "1" digits).