Allow public terraform modules to anonymous users

What does this MR do and why?

If the user does not send a token, there's no need to try to authenticate it. So specifying a blank token should be enough.

We could probably not even concatenate the token. It might actually be neater. Or maybe it's better to keep sending a blank token parameter as a response, just so we keep consistent on the API response always having a token in the URL. I've opted for the latter. I'm open minded to change it, though.

Relates to: #363268 (closed)

Screenshots or screen recordings

Private projects return the following when running terraform init:

> terraform init
Initializing modules...
╷
│ Error: Module has no versions
│
│ Module "gdk.test:3443/h5bp/random-pet/local" (config.tf:1) has no versions available on gdk.test:3443.
╵

How to set up and validate locally

In your GDK, create or use a public project in a certain group:

1. Create your sample modules/random-pet/main.tf:

terraform {
  required_providers {
      random = {
        source  = "hashicorp/random"
      version = "3.1.2"
    }
  }
}

provider "random" {}

resource "random_pet" "random_pet" {
  length = var.length
}

2. Create your sample modules/random-pet/outputs.tf:

output "random_pet" {
  value = random_pet.random_pet.id
}

3. Create your sample modules/random-pet/variables.tf:

variable "length" {
  default = 1
  type    = number
}  

4. Create your .gitlab-ci.yml to publish te module, and validate that the pipeline has published it afterwards:

stages:
  - upload-test-module

  upload:
    stage: upload-test-module
    script:
      - cd modules/random-pet
      - tar -cvzf ${CI_PROJECT_NAME}-local-${TERRAFORM_MODULE_VERSION}.tgz -C ${CI_PROJECT_DIR} .
      - 'curl -k --header "JOB-TOKEN: ${CI_JOB_TOKEN}" --upload-file ${CI_PROJECT_NAME}-local-${TERRAFORM_MODULE_VERSION}.tgz ${CI_API_V4_URL}/projects/${CI_PROJECT_ID}/packages/terraform/modules/random-pet/local/${TERRAFORM_MODULE_VERSION}/file'
    variables:
      TERRAFORM_MODULE_VERSION: 0.0.1
    rules:
      - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'  

5. On your local environment, create a file to install the, e.g.: config.tf:

Note that you should replace your GDK instance with your own hostname. In my case I have gdk.test running with https enabled on port 3443.

module "my_module_name" {
  source = "gdk.test:3443/h5bp/random-pet/local"
  version = "0.0.1"
}  

6. Run terraform init locally, and verify that you don't get a 401.

MR acceptance checklist

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