fix(ecs): allow empty placementStrategies on EC2Service (#35580)

### Reason for this change

You can't remove placement strategies from an ECS service using CDK once they've been set.

### Description of changes

Same fix as #30382 but for `placementStrategies`. Apparently this was not done at that time because CloudFormation [previously did not have the appropriate semantics](#27572 (comment)) for setting PlacementStrategies to the empty array as it did for PlacementConstraints, but that is no longer the case. The CloudFormation docs [state](https://docs.aws.amazon.com/AWSCloudFormation/latest/TemplateReference/aws-resource-ecs-service.html#cfn-ecs-service-placementstrategies):
> To remove this property from your service resource, specify an empty PlacementStrategy array.

### Describe any new or updated permissions being added

N/A

### Description of how you validated changes

Unit and integration tests.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: isker