In the USA, the power of presidential pardon includes the power to offer amnesties. And amnesties certainly can be given to people who have never been convicted.
Pardon = the defendant is forgiven his crime and released from prison, but the fact of being guilty of the crime remains, i.e., the defendant is forgiven for his bad acts.
Amnesty = the defendant is cleared of legal guilt for the crime and the fact of having committed the crime is also cleared, i.e., it is as if the defendant never committed a crime at all.
Pre-emptive amnesty is more simply referred to as "immunity."
