[#164] sdk/object: Return nil parent if parent fields are empty

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/object/raw_test.go

@@ -186,8 +186,13 @@ func TestRawObject_SetChildren(t *testing.T) {
 func TestRawObject_SetParent(t *testing.T) {
 	obj := NewRaw()
 
+	require.Nil(t, obj.GetParent())
+
 	par := NewRaw()
 	par.SetID(randID(t))
+	par.SetContainerID(container.NewID())
+	par.SetSignature(pkg.NewSignature())
+
 	parObj := par.Object()
 
 	obj.SetParent(parObj)

pkg/object/rw.go

@@ -273,10 +273,17 @@ func (o *rwObject) GetParent() *Object {
 		GetHeader().
 		GetSplit()
 
+	parSig := h.GetParentSignature()
+	parHdr := h.GetParentHeader()
+
+	if parSig == nil && parHdr == nil {
+		return nil
+	}
+
 	oV2 := new(object.Object)
 	oV2.SetObjectID(h.GetParent())
-	oV2.SetSignature(h.GetParentSignature())
-	oV2.SetHeader(h.GetParentHeader())
+	oV2.SetSignature(parSig)
+	oV2.SetHeader(parHdr)
 
 	return NewFromV2(oV2)
 }