Return null instead of an empty tftypes.List when the list is empty

[gdavison]

Empty lists returned by the provider were causing errors on any subsequent state refresh. The state stored a null value, whereas the generated plan would return an empty tftypes.List. This would cause Terraform to fail with an error like

│ Error: Provider produced invalid plan
│ 
│ Provider "registry.terraform.io/hashicorp/awscc" planned an invalid value for awscc_ec2_vpc.test.tags: planned value
│ cty.ListValEmpty(cty.Object(map[string]cty.Type{"key":cty.String, "value":cty.String})) for a non-computed attribute.
│ 
│ This is a bug in the provider, which should be reported in the provider's own issue tracker.

This PR short-circuits the empty list case to return a null value of the list type instead of an empty list. This allows Terraform to continue and encounter behaviour such as seen in #301, where default values are not handled as expected because fields that should have default values or be computed are not marked as such in the Cloud Control schemas.

Terraform configuration

terraform {
  required_providers {
    awscc = {
      source  = "hashicorp/awscc"
    }
  }
}

provider "awscc" {
  region = "us-west-2"
}

resource "awscc_ec2_vpc" "test" {
  cidr_block = "10.0.0.0/16"
}

Before the update, running terraform apply and then terraform plan results in the error above.

After the update, the terraform plan succeeds, but shows the following diff

Note: Objects have changed outside of Terraform

Terraform detected the following changes made outside of Terraform since the last "terraform apply":

  # awscc_ec2_vpc.test has changed
  ~ resource "awscc_ec2_vpc" "test" {
      + enable_dns_hostnames    = false
      + enable_dns_support      = true
        id                      = "vpc-056c110e94f3ed041"
      + instance_tenancy        = "default"
        # (5 unchanged attributes hidden)
    }


Unless you have made equivalent changes to your configuration, or ignored the relevant attributes using ignore_changes, the following plan may include actions to
undo or respond to these changes.

─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # awscc_ec2_vpc.test will be updated in-place
  ~ resource "awscc_ec2_vpc" "test" {
      - enable_dns_hostnames    = false -> null
      - enable_dns_support      = true -> null
        id                      = "vpc-056c110e94f3ed041"
      - instance_tenancy        = "default" -> null
        # (5 unchanged attributes hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.

Note that a second terraform apply still results in the error as reported in #301.

Closes #342
Closes #351
Closes #324

[ewbankkit]

LGTM 🚀.