using spot_instance_types returns InvalidParameterValue: Invalid value '' for InstanceType.

_This issue was originally opened by @lmayorga1980 in https://github.com/hashicorp/packer/issues/11585 and has been migrated to this repository. The original issue description is below._

---

#### Overview of the Issue

I get an error trying to use `spot_instances_types` feature.

#### Reproduction Steps

Run a Packer Template with the following attributes.

```
...
"spot_instance_types": "t2.small,t2.medium",
"spot_price": "auto",
...
```

### Packer version

`1.7.9`

### Operating system and Environment details

CentOS 7.x

### Log Fragments and crash.log files

```
==> amazon-ebs: Error launching source instance: InvalidParameterValue: Invalid value '' for InstanceType.
==> amazon-ebs: status code: 400, request id: 0f47da10-0c42-4a40-9a86-0afd3560bee5
==> amazon-ebs: Pausing before cleanup of step 'StepCleanupVolumes'. Press enter to continue.
==> amazon-ebs: No volumes to clean up, skipping
==> amazon-ebs: Pausing before cleanup of step 'StepIamInstanceProfile'. Press enter to continue.
==> amazon-ebs: Pausing before cleanup of step 'StepSecurityGroup'. Press enter to continue.
==> amazon-ebs: Pausing before cleanup of step 'StepKeyPair'. Press enter to continue.
==> amazon-ebs: Deleting temporary keypair...
==> amazon-ebs: Pausing before cleanup of step 'StepNetworkInfo'. Press enter to continue.
==> amazon-ebs: Pausing before cleanup of step 'StepSourceAMIInfo'. Press enter to continue.
==> amazon-ebs: Pausing before cleanup of step 'StepPreValidate'. Press enter to continue.
Build 'amazon-ebs' errored after 16 seconds 413 milliseconds: Error launching source instance: InvalidParameterValue: Invalid value '' for InstanceType.
status code: 400, request id: 0f47da10-0c42-4a40-9a86-0afd3560bee5

==> Wait completed after 16 seconds 413 milliseconds

==> Some builds didn't complete successfully and had errors:
--> amazon-ebs: Error launching source instance: InvalidParameterValue: Invalid value '' for InstanceType.
status code: 400, request id: 0f47da10-0c42-4a40-9a86-0afd3560bee5
```

I am not providing **Instance Type** value but the documentation mentions that both Instance Type and Spot Instance Types are exclusive.