如何在spring云合约中指定可选的Date属性？

import org.springframework.lang.Nullable;

public class ContractRequest {
    @Nullable
    private Date optionalDate;

    // getter and setter
}

package contracts

import org.springframework.cloud.contract.spec.Contract

Contract.make {
    request {
        body(
            optionalDate: $(optional(date()))
        )
    }
    response {
        status 200
    }
}