fix: Create private_ipv6_egress routes only when having at least one private subnet (#1062)

Co-authored-by: Manuel Enrique Colotti <manuel.colotti@agilelab.it>

main.tf

@@ -1021,7 +1021,7 @@ resource "aws_egress_only_internet_gateway" "this" {
 }
 
 resource "aws_route" "private_ipv6_egress" {
-  count = local.create_vpc && var.create_egress_only_igw && var.enable_ipv6 ? local.nat_gateway_count : 0
+  count = local.create_vpc && var.create_egress_only_igw && var.enable_ipv6 && local.len_private_subnets > 0 ? local.nat_gateway_count : 0
 
   route_table_id              = element(aws_route_table.private[*].id, count.index)
   destination_ipv6_cidr_block = "::/0"