Can a retail store ask me to empty my pockets to prove I didn't take anything?

Yes. If a merchant has reasonable cause to believe someone is shoplifting in their store, the law says they can detain the person for a reasonable time in order to question the suspected shoplifter and/or call the police. The merchant can certainly ask you to empty your pockets but you can choose not to cooperate. They would then most likely assume you did steal something and call the police. Depending on the information which led the store to believe you had been shoplifting, the police might arrested you and conduct a search incident to the arrest.