[#302] pkg/owner: Document default values set in NewID

Document field values of instance constructed via `NewID`.
Assert the values in corresponding unit test.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/owner/id.go

@@ -25,6 +25,9 @@ func NewIDFromV2(idV2 *refs.OwnerID) *ID {
 // NewID creates and initializes blank ID.
 //
 // Works similar as NewIDFromV2(new(OwnerID)).
+//
+// Defaults:
+//  - value: nil.
 func NewID() *ID {
 	return NewIDFromV2(new(refs.OwnerID))
 }

pkg/owner/id_test.go

@@ -116,3 +116,22 @@ func TestID_ToV2(t *testing.T) {
 		require.Nil(t, x.ToV2())
 	})
 }
+
+func TestID_String(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		id := NewID()
+
+		require.Empty(t, id.String())
+	})
+}
+
+func TestNewID(t *testing.T) {
+	t.Run("default values", func(t *testing.T) {
+		id := NewID()
+
+		// convert to v2 message
+		idV2 := id.ToV2()
+
+		require.Nil(t, idV2.GetValue())
+	})
+}