terraform optional keys in maps

optional-maps.tf

locals {
  required_value = {
    RequiredValue = "${var.required}"
  }

  optional_value = {
    exists = {
      OptionalValue = "${var.optional}"
    }
    not_exists = {}

  }
  final_value = "${merge(
    local.required_value,
    local.optional_value[var.optional != "" ? "exists" : "not_exists"]
  )}"
}