[Bug] Resource 'elasticstack_fleet_package_policy' does not permit policies with zero inputs

[markjandejong]

Describe the bug
The resource elasticstack_fleet_package_policy does not allow the deployment of package policies with zero inputs; specifically a package policy for the Elastic Agent package. The Elastic Agent package requires no configuration, thus no inputs. This could be considered a feature request since the behavior is expected, but I have issued it as a bug since the resource doe not handle all possible scenarios.

    {
      "id": "78d6f340-2fd0-11ee-8e61-b7fe88cXXXXX",
      "version": "WzE5MDE3LDdd",
      "name": "elastic_agent-1",
      "namespace": "default",
      "description": "",
      "package": {
        "name": "elastic_agent",
        "title": "Elastic Agent",
        "version": "1.10.0"
      },
      "enabled": true,
      "policy_id": "b2c90fc0-2fa3-11ee-8e61-b7fe88c46c49",
      "inputs": [],
      "revision": 2,
      "created_at": "2023-07-31T18:31:32.840Z",
      "created_by": "XXXX",
      "updated_at": "2023-07-31T19:14:10.250Z",
      "updated_by": "XXXX"
    },

To Reproduce
Steps to reproduce the behavior:

1. TF configuration used '...'

resource "elasticstack_fleet_package_policy" "sample" {
  agent_policy_id = var.agent_policy_id
  dynamic input {
    for_each = local.inputs
    content {
        enabled      = input.value.enabled
        input_id     = input.value.input_id
        streams_json = input.value.streams_json
        vars_json    = input.value.vars_json
    }
  }
  name            = var.name
  namespace       = var.namespace
  package_name    = var.package_name
  package_version = var.package_version

  description = var.description
  enabled     = var.enabled
  force       = var.force
  policy_id   = var.policy_id
  vars_json   = local.vars_json
}

Where local.inputs is an empty list

2. TF operations to execute to get the error '...' [e.g terraform plan,terraform apply, terraform destroy]
   terraform plan
3. See the error in the output '...'

╷
│ Error: Insufficient input blocks
│ 
│   on modules/fleet_package_policy/main.tf line 1, in resource "elasticstack_fleet_package_policy" "sample":
│    1: resource "elasticstack_fleet_package_policy" "sample" {
│ 
│ At least 1 "input" blocks are required.
╵
ERRO[0021] 1 error occurred:
        * exit status 1

This has also been attempted with an empty input block which results in the following:

╷
│ Error: Missing required argument
│ 
│   on modules/fleet_package_policy/main.tf line 14, in resource "elasticstack_fleet_package_policy" "sample":
│   14:     content {}
│ 
│ The argument "input_id" is required, but no definition was found.
╵
ERRO[0024] 1 error occurred:
        * exit status 1

Expected behavior
A clear and concise description of what you expected to happen.
The resource be deployed without any inputs defined
Debug output
Run terraform command with TF_LOG=trace and provide extended information on TF operations.

Screenshots
If applicable, add screenshots to help explain your problem.

Versions (please complete the following information):

• OS: [e.g. Linux]
• Terraform Version [e.g. 1.0.0]
• Provider version [e.g. v0.1.0]
• Elasticsearch Version [e.g. 7.16.0]

Additional context
Add any other context about the problem here.

[tobio]

@taylor-swanson if you get a chance to take a look here.