[semver:minor] Always allow platform-version overrides

[lokst]

Resolves #121

Change the default value for the platform-version command/job parameter from LATEST to '', and only provide the --platform-version argument to the AWS CLI when platform-version is non-blank.

The AWS CLI docs for --platform-version (https://docs.aws.amazon.com/cli/latest/reference/ecs/run-task.html) state:

If one is not specified, the LATEST platform version is used by default

Hence, this PR should generally be a backwards-compatible change since the AWS CLI already uses LATEST as the default when no --platform-version argument is provided to the CLI anyway, so not passing --platform-version (which will be the new orb default behavior) should have the same effect as explicitly setting --platform-version LATEST when the launch type is FARGATE, as the orb would do in the past.

[dsayling]

If launch type isn't fargate and they set the string, will something unexpected happen?

[lokst]

If launch type isn't fargate and they set the string, will something unexpected happen?

I think the CLI would show an error message, but I'll test it out to confirm

[lokst]

@dsayling Tested this out with an EC2 launch-type. The CLI does perform some validation on the --platform-version argument 🎉

aws ecs register-task-definition \
              --family ecs-orb-ec2-1-sleep360 \
              --cpu 256 --memory 512 \
              --container-definitions "[{\"name\":\"sleep\",\"image\":\"busybox\",\"command\":[\"sleep\",\"360\"],\"memory\":256,\"essential\":true}]" --region us-east-1

aws ecs run-task --platform-version "LATEST" --cluster ecs-orb-ec2-1-cluster --launch-type EC2 --task-definition ecs-orb-ec2-1-sleep360:114 --region us-east-1

An error occurred (InvalidParameterException) when calling the RunTask operation: The platform version must be null when specifying an EC2 launch type.

[lokst]

The changes are now available in circleci/aws-ecs@2.1.0