core: Avoid crash on empty TypeSet blocks (#14305)

helper/schema/serialize.go

@@ -85,6 +85,9 @@ func SerializeValueForHash(buf *bytes.Buffer, val interface{}, schema *Schema) {
 // to hash complex substructures when used in sets, and so the serialization
 // is not reversible.
 func SerializeResourceForHash(buf *bytes.Buffer, val interface{}, resource *Resource) {
+	if val == nil {
+		return
+	}
 	sm := resource.Schema
 	m := val.(map[string]interface{})
 	var keys []string

helper/schema/set_test.go

@@ -111,3 +111,20 @@ func TestSetUnion(t *testing.T) {
 func testSetInt(v interface{}) int {
 	return v.(int)
 }
+
+func TestHashResource_nil(t *testing.T) {
+	resource := &Resource{
+		Schema: map[string]*Schema{
+			"name": {
+				Type:     TypeString,
+				Optional: true,
+			},
+		},
+	}
+	f := HashResource(resource)
+
+	idx := f(nil)
+	if idx != 0 {
+		t.Fatalf("Expected 0 when hashing nil, given: %d", idx)
+	}
+}