fix(terraform): guard NAT gateway outputs against disabled NAT

[Copilot]

Terraform outputs for NAT gateway IDs and IPs were directly indexing arrays without checking if NAT gateways were enabled, causing plan failures when enable_nat_gateway=false.

Changes

• Wrapped nat_public_ips[0] and natgw_ids[0] with try() to return null when NAT is disabled
• Updated output descriptions to document null fallback behavior

# Before
output "nat_gateway_id" {
  value = module.vpc.natgw_ids[0]  # Errors if list is empty
}

# After  
output "nat_gateway_id" {
  value       = try(module.vpc.natgw_ids[0], null)
  description = "ID of the NAT Gateway (null if NAT gateway is disabled)"
}

Addresses feedback in #37 (comment)

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