locals {
  map1 = {
    bad = null
    good = false
  }
  map2 = {
    bad = "overwrite"
    good = true
  }
}

output "example" {
  value = provider::deepmerge::mergo(local.map1, local.map2)
}

$ terraform plan
╷
│ Error: Error in function call
│ 
│   on main.tf line 44, in output "example":
│   44:   value = provider::deepmerge::mergo(local.map1, local.map2)
│     ├────────────────
│     │ while calling provider::deepmerge::mergo(maps...)
│     │ local.map1 is object with 2 attributes
│     │ local.map2 is object with 2 attributes
│ 
│ Call to function "provider::deepmerge::mergo" failed: Error unmarshalling argument 1: invalid character '<' looking for beginning of value.
╵

variable "policies" {
  type = map(map(object({
    name         = optional(string)
    policy_arn   = optional(string)
    })
  ))
}

policies = {
  group1 = {
    policy1 = {
      name = "something"
    }
  }
}

policies = {
  group1 = {
    policy1 = {
      policy_arn = "arn"
    }
  }
}

policies = {
  group1 = {
    policy1 = {
      name        = null
      policy_arn = "arn"
    }
  }
}