Error: while setting policy (), encountered: Error parsing policy: Unknown error parsing statement

Problem

An aws_iam_policy_document data source with no statement blocks produces a document that can't be read by aws_iam_policy. 

Cause

First an apply occurs with a valid statement and is saved to the state file.

data "aws_iam_policy_document" "maybe_empty" {
statement {
actions = [ "ssm:SendCommand" ]
resources = [ "*" ]
}
}

resource "aws_iam_policy" "example" {
name = "example"
policy = data.aws_iam_policy_document.maybe_empty.json
}

Changes are then made to the Terraform configuration and statements are removed. 

data "aws_iam_policy_document" "maybe_empty" {
# statement {
# actions = [ "ssm:SendCommand" ]
# resources = [ "*" ]
# }
}

resource "aws_iam_policy" "example" {
name = "example"
policy = data.aws_iam_policy_document.maybe_empty.json
}

With an empty statement, the state file terraform state show data.aws_iam_policy_document.maybe_empty will have the policy for both resources set to:

{
"Version": "2012-10-17",
"Statement": null
}

On the next plan, when trying to update and compare a policy already applied to a new one that has no statement or path configured, that policy will be read, but will fail with Error: while setting policy (), encountered: Error parsing 1 policy: Unknown error parsing statement.

Solution:

The IAM policy needs to be removed from the state file and re-imported, and the policy document needs at least one statement.

• Connect to the remote backend
• Run terraform init to initialize the working directory and allow Terraform to copy the remote state locally.
• Locate the resource and run terraform state rm aws_iam_policy.$policy_nameto remove the resource from the Terraform state file. 
• Locate the resource aws_iam_policy_document and run terraform state rm data.aws_iam_policy_document.$policy_nameto remove the resource from the Terraform state file. 
• Ensure that the policy document has a statement in it.
• Run terraform import to import both resources back into the state file.
• Run terraform plan and terraform apply.