Empty block syntactic sugar

Hi,

I've noticed that often configuration files contain empty blocks, for example:

```
variable "prod_access_key" {}
variable "prod_secret_key" {}
variable "test_account_id" {}
variable "test_access_key" {}
variable "test_secret_key" {}
variable "bucket_name" {}
```
taken from: https://github.com/terraform-providers/terraform-provider-aws/blob/24e937a299c9115be47052404a97d6c9d5a4ccfa/examples/s3-cross-account-access/variables.tf.

Have you considered in HCL2 allowing shorter form without explicit mention the empty block? For example:
```
variable "prod_access_key"
variable "prod_secret_key"
variable "test_account_id"
variable "test_access_key"
variable "test_secret_key"
variable "bucket_name"
```

Thanks!
Hadar

Review the HCL2 block syntax and the referenced examples/s3-cross-account-access/variables.tf configuration to understand current empty-block usage. Determine whether bare two-label declarations should be accepted, then define the syntax and compatibility behavior before adding parser coverage; done means the proposal has an agreed design and passing tests.

Written by the indexing model from the issue text.