Merge two maps on a conditional

I am working on tagging AWS resources, I want only few resources have optional_tags. Optional_tags variables are only passed if available. All variables declared in optional_tags are default null.

I have tried using tags and all vars in that but the for loop fails on null values.

Error: Missing required argument
  on line 41, in resource "aws_ec2_tag" "ami-tagging":
  41:   value       = each.value

Now I am trying to use two maps and merge on conditional as below. If value exist for a optional_tag variable then merge with it or merge with itself. It has been complaining about missing ( . I am very new to terraform, any help is appreciated.

locals {

  optional_tags = {
    docker_version     = var.docker_version
    kubernetes         = var.k8s_version
    cni_plugin_version = var.cni_plugin_version
    containerd_version = var.containerd_version
    target             =

  default_tags = {
    Description   = var.ami_description
    Name          = var.ami_name
    Creator       = var.creator
  tags =  merge
    local.optional_tags[contains(keys(local.optional_tags.containerd_version),local.optional_tags) ? local.default_tags]


resource "aws_ec2_tag" "ami-taggging" {
  resource_id =
  for_each    = local.tags
  key         = each.key
  value       = each.value