fix(ecs): removed omitEmptyArray for placementConstraints (#27555) (#30382)

### Issue 

fixes #27555 
Closes Half Fix [i.e, Allows user to give Empty Placement Constraints ]  #27555 

This PR does not address supporting empty placement strategies because of the following reason : [27555 : comment](#27572 (comment))

This was raised with the guidance from - [pr / 28431 : Comment ](#28431 (comment))

### Reason for this change

Users unable to give empty placementConstraints

### Description of how you validated changes

- Added a UnitCase to cover with empty `[]` placementConstraints
- Integration Tests
```
$ yarn integ test/aws-ecs/test/ec2/integ.placement-constraint-default-empty.js --update-on-failed
```
After integ tests were completed, `npm test` to verify the snapshot.

### 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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Author: stm29