Allow AMI block device mapping to be created without snapshot

_This issue was originally opened by @otterley as hashicorp/packer#5290. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

I have a use case where I have two `launch_block_device_mappings`, `/dev/sda1` (root device) and `/dev/sdb` (data device). The data device exists during the build phase to satisfy some build-time prerequisites, but the actual volume content can be discarded at the the end of the build. The device can also be quite small.

Meanwhile, there are two `ami_block_device_mappings` of the same device names, `/dev/sda1` and `/dev/sdb`. I'd like `/dev/sdb` NOT to be associated with a snapshot, since at launch time the instance should have a fresh volume. But Packer seems to always associate it with a snapshot because the device was mapped at launch time. Setting `snapshot_id` to `""` seems to have no effect.