tftypes: add tftypes.Value.IsFullyNull()

[bbasata]

Related Issue

For hashicorp/terraform-plugin-framework#1193, to fix hashicorp/terraform-plugin-framework#1184.

Description

This pull request adds a method to tftypes.Value that returns true for an object that has all null attributes. In this way, IsNull/IsFullyNull mirror the existing IsKnown/IsFullyKnown.

For example, the equivalent tftypes.Value of this value is "fully null":

"identity": { // <---------------- all identity attributes are null
    "account_id": null,
    "bucket": null,
    "region": null
}

This introduces no behavior changes to terraform-plugin-go. It provides a type system utility for SDKs to consume. For example, terraform-plugin-framework can use tftypes.Value.IsFullyNull() to reject "fully null" resource identity values from RPC responses.

cc: @austinvalle

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

No.

[austinvalle]

Code itself looks good, I did have one question. Can we also add a changelog for this?

[bbasata]

cc: @dbanck

In Plugin SDK and Framework, we will return an error diagnostic when a null or "empty" resource identity would be returned for a resource that has an identity schema.

As defined here, "empty" (or IsFullyNull) means that the value is a null primitive or the value is an aggregate that consists of only empty elements and attributes. So it's "deeply empty" 😢. Any aggregate of lists of null/empty values, objects of null/empty values, etc.

How well does this agree with Terraform?