[#302] pkg/object: Document default values set in NewSplitInfo

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

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

pkg/object/splitinfo.go

@@ -13,6 +13,12 @@ func NewSplitInfoFromV2(v2 *object.SplitInfo) *SplitInfo {
 	return (*SplitInfo)(v2)
 }
 
+// NewSplitInfo creates and initializes blank SplitInfo.
+//
+// Defaults:
+//  - splitID: nil;
+//  - lastPart nil;
+//  - link: nil.
 func NewSplitInfo() *SplitInfo {
 	return NewSplitInfoFromV2(new(object.SplitInfo))
 }

pkg/object/splitinfo_test.go

@@ -68,3 +68,21 @@ func TestSplitInfo_ToV2(t *testing.T) {
 		require.Nil(t, x.ToV2())
 	})
 }
+
+func TestNewSplitInfo(t *testing.T) {
+	t.Run("default values", func(t *testing.T) {
+		si := object.NewSplitInfo()
+
+		// check initial values
+		require.Nil(t, si.SplitID())
+		require.Nil(t, si.LastPart())
+		require.Nil(t, si.Link())
+
+		// convert to v2 message
+		siV2 := si.ToV2()
+
+		require.Nil(t, siV2.GetSplitID())
+		require.Nil(t, siV2.GetLastPart())
+		require.Nil(t, siV2.GetLink())
+	})
+}