Registration of accounts without signatories

[mversic]

Currently we seem to allow this, but we don't allow burning the last signatory of an account (we have a test ensuring this public_keys_cannot_be_burned_to_nothing). Should we also prevent creation of Account/NewAccount without signatories. If so, then I propose to make modifications to all the constructors Account::new/Decode/Deserialize to only allow creation of Accounts with at least one signatory

This might be a potentially API changing modification