provider/aws: Fix missing AMI issue with Launch Configurations

[catsby]

If a launch configuration references an AMI that does not exist, it will fail to find the DeviceName for the AMI when it's building up it's Block Device Mappings.

Unlike other invocations of the fetchRootDeviceName method, Launch Configurations was not checking for a nil device name.

The resulting Block Mappings end up something like this:

BlockDeviceMappings: [{
       Ebs: {
         DeleteOnTermination: true,
         VolumeSize: 1024,
         VolumeType: "gp2"
       }
     }],

(no device name, required for EBS)

And then get an error on apply similar to this:

* aws_launch_configuration.lc: Error creating launch configuration: InvalidParameter: 1 validation errors:
- missing required parameter: BlockDeviceMappings[0].DeviceName

This PR errors if a nil device name is returned.

[stack72]

how did you find this error @catsby? it wasn't even reported 😛

[catsby]

It's edge-case – the AMI needs to be found in fetchRootDeviceName but be in the process of deregistering. Unlikely to happen often, but still, we should account for that nil

[jen20]

LGTM - nasty edge case there

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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