terraform: Fix createEmptyBlocks NestingSingle bug

When calling createEmptyBlocks, we only intend to process legacy SDK
blocks (NestingList or NestingSet), but the function also needs to pass
through NestingSingle block types untouched. To do so we must only look
up the container's element type after we've established that the block
is backed by a container.

terraform/eval_read_data_plan.go

@@ -239,8 +239,6 @@ func createEmptyBlocks(schema *configschema.Block, val cty.Value) cty.Value {
 			continue
 		}
 
-		ety := block.Type().ElementType()
-
 		// helper to build the recursive block values
 		nextBlocks := func() []cty.Value {
 			// this is only called once we know this is a non-null List or Set
@@ -259,6 +257,7 @@ func createEmptyBlocks(schema *configschema.Block, val cty.Value) cty.Value {
 		// We are only concerned with block types that can come from the legacy
 		// sdk, which means TypeList or TypeSet.
 		case configschema.NestingList:
+			ety := block.Type().ElementType()
 			switch {
 			case block.IsNull():
 				objMap[name] = cty.ListValEmpty(ety)
@@ -269,6 +268,7 @@ func createEmptyBlocks(schema *configschema.Block, val cty.Value) cty.Value {
 			}
 
 		case configschema.NestingSet:
+			ety := block.Type().ElementType()
 			switch {
 			case block.IsNull():
 				objMap[name] = cty.SetValEmpty(ety)

terraform/eval_read_data_plan_test.go

@@ -86,6 +86,22 @@ func TestReadDataCreateEmptyBlocks(t *testing.T) {
 		},
 	}
 
+	singleSchema := &configschema.Block{
+		BlockTypes: map[string]*configschema.NestedBlock{
+			"single": {
+				Nesting: configschema.NestingSingle,
+				Block: configschema.Block{
+					Attributes: map[string]*configschema.Attribute{
+						"attr": {
+							Type:     cty.String,
+							Optional: true,
+						},
+					},
+				},
+			},
+		},
+	}
+
 	for _, tc := range []struct {
 		name   string
 		schema *configschema.Block
@@ -330,6 +346,20 @@ func TestReadDataCreateEmptyBlocks(t *testing.T) {
 				}),
 			}),
 		},
+		{
+			"single-block-null",
+			singleSchema,
+			cty.ObjectVal(map[string]cty.Value{
+				"single": cty.NullVal(cty.Object(map[string]cty.Type{
+					"attr": cty.String,
+				})),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"single": cty.NullVal(cty.Object(map[string]cty.Type{
+					"attr": cty.String,
+				})),
+			}),
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			val := createEmptyBlocks(tc.schema, tc.val)