Fix VPC parameter validation and security group reference for existing VPC deployment

[syobochim]

Issue #, if available:
N/A

Description of changes:
This PR fixes VPC deployment issues when using existing VPC infrastructure by addressing parameter validation and security group VPC reference problems.

Changes Made:

• VPC Parameter Validation Fix: Changed VpcCidrBlock parameter minLength from 9 to 0 in distributed-load-testing-on-aws-stack.ts to allow empty input when using existing VPC
• Security Group VPC Reference Fix: Added conditional VPC reference for ECS security group using CloudFormation Fn::If in ecs.ts to properly handle both new and existing VPC scenarios

Problem Solved:

• Resolves deployment failures when using existing VPC due to parameter validation requiring non-empty CIDR block
• Fixes security group VPC reference issues in existing VPC deployment scenarios
• Ensures proper conditional logic for both new VPC creation and existing VPC usage

Files Modified:

• source/infrastructure/lib/distributed-load-testing-on-aws-stack.ts - VPC parameter validation fix (1 line changed)
• source/infrastructure/lib/testing-resources/ecs.ts - Security group VPC reference fix (9 lines added)

Checklist

• 👋 I have run the unit tests, and all unit tests have passed.
• ⚠️ This pull request might incur a breaking change.

[JimTharioAmazon]

This PR has been implemented in v3.4.1. Thank you.