Guard NAT gateway outputs against empty lists

[Copilot]

Addresses feedback on PR #37 where NAT gateway outputs directly index arrays without guards, causing Terraform to fail if NAT is disabled or returns empty lists.

Changes:

• Wrapped module.vpc.nat_public_ips[0] and module.vpc.natgw_ids[0] with try(..., null)
• Added descriptions clarifying outputs return null when NAT is disabled

Example:

# Before
output "vpc_nat_gateway_static_public_ip" {
  value = module.vpc.nat_public_ips[0]  # Fails if empty
}

# After
output "vpc_nat_gateway_static_public_ip" {
  value       = try(module.vpc.nat_public_ips[0], null)
  description = "Static public IP of the NAT Gateway (null if NAT is disabled)"
}

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