internal/toproto*: Convert null list/set block values to empty (#621)

Reference: #604
Reference: #620

This logic implements the inverse of what was added to `internal/fromproto*` where empty block values are converted to null. While Terraform implements some logic to automatically convert these for state responses from `ReadResource` and others, the same null values can cause errors with Terraform's plan validity logic.

This fixes the potential new error caused by only introducing the `internal/fromproto*` logic, which was not released and why it does not contain a changelog entry in that regard, However, this change is considered an overall enhancement prior and including those changes since it means the provider developers will no longer be burdened with understanding Terraform's implementation details with null versus empty collection blocks when setting values for responses.

.changelog/621.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+tfsdk: Automatically prevented Terraform `nested blocks must be empty to indicate no blocks` errors for responses containing `Plan` and `State` types
+```

internal/fromproto5/dynamic_value.go

@@ -17,8 +17,6 @@ import (
 // developers from needing to understand Terraform's differences between
 // block and attribute values where blocks are technically never null, but from
 // a developer perspective this distinction introduces unnecessary complexity.
-// This null block value translation is automatically handled by Terraform from
-// provider responses.
 func DynamicValue(ctx context.Context, proto5 *tfprotov5.DynamicValue, schema fwschema.Schema, description fwschemadata.DataDescription) (fwschemadata.Data, diag.Diagnostics) {
 	var diags diag.Diagnostics
 

internal/fromproto6/dynamic_value.go

@@ -17,8 +17,6 @@ import (
 // developers from needing to understand Terraform's differences between
 // block and attribute values where blocks are technically never null, but from
 // a developer perspective this distinction introduces unnecessary complexity.
-// This null block value translation is automatically handled by Terraform from
-// provider responses.
 func DynamicValue(ctx context.Context, proto6 *tfprotov6.DynamicValue, schema fwschema.Schema, description fwschemadata.DataDescription) (fwschemadata.Data, diag.Diagnostics) {
 	var diags diag.Diagnostics
 

internal/fwschemadata/data_nullify_collection_blocks.go

@@ -10,7 +10,7 @@ import (
 )
 
 // NullifyCollectionBlocks converts list and set block empty values to null
-// values.
+// values. The reverse conversion is ReifyNullCollectionBlocks.
 func (d *Data) NullifyCollectionBlocks(ctx context.Context) diag.Diagnostics {
 	var diags diag.Diagnostics
 

internal/fwschemadata/data_reify_null_collection_blocks.go

@@ -0,0 +1,46 @@
+package fwschemadata
+
+import (
+	"context"
+
+	"github.com/hashicorp/terraform-plugin-framework/diag"
+	"github.com/hashicorp/terraform-plugin-framework/internal/fromtftypes"
+	"github.com/hashicorp/terraform-plugin-framework/internal/fwschema"
+	"github.com/hashicorp/terraform-plugin-go/tftypes"
+)
+
+// ReifyNullCollectionBlocks converts list and set block null values to empty
+// values. This is the reverse conversion of NullifyCollectionBlocks.
+func (d *Data) ReifyNullCollectionBlocks(ctx context.Context) diag.Diagnostics {
+	var diags diag.Diagnostics
+
+	blockPathExpressions := fwschema.SchemaBlockPathExpressions(ctx, d.Schema)
+
+	// Errors are handled as richer diag.Diagnostics instead.
+	d.TerraformValue, _ = tftypes.Transform(d.TerraformValue, func(tfTypePath *tftypes.AttributePath, tfTypeValue tftypes.Value) (tftypes.Value, error) {
+		// Only transform null values.
+		if !tfTypeValue.IsNull() {
+			return tfTypeValue, nil
+		}
+
+		fwPath, fwPathDiags := fromtftypes.AttributePath(ctx, tfTypePath, d.Schema)
+
+		diags.Append(fwPathDiags...)
+
+		// Do not transform if path cannot be converted.
+		// Checking against fwPathDiags will capture all errors.
+		if fwPathDiags.HasError() {
+			return tfTypeValue, nil
+		}
+
+		// Do not transform if path is not a block.
+		if !blockPathExpressions.Matches(fwPath) {
+			return tfTypeValue, nil
+		}
+
+		// Transform to empty value.
+		return tftypes.NewValue(tfTypeValue.Type(), []tftypes.Value{}), nil
+	})
+
+	return diags
+}