如何在Mapstruct中将空字符串转换为Long？

import org.mapstruct.Mapper;
import org.mapstruct.Mapping;
import org.mapstruct.factory.Mappers;

@Mapper
public interface StringToLongConverter {

    StringToLongConverter INSTANCE = Mappers.getMapper(StringToLongConverter.class);

    @Mapping(target = "targetField", expression = "java(convertToLong(source.getSourceField()))")
    TargetClass convert(SourceClass source);

    default Long convertToLong(String value) {
        if (value == null || value.isEmpty()) {
            return null;
        }
        return Long.parseLong(value);
    }
}