[#302] pkg/object: Convert nil `Tombstone` to nil message

Document that `NewTombstoneFromV2` function return
`nil` when called on `nil`. Write corresponding
unit tests.

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

pkg/object/splitid_test.go

@@ -45,3 +45,4 @@ func TestSplitID(t *testing.T) {
 		})
 	})
 }
+

pkg/object/tombstone.go

@@ -9,6 +9,8 @@ import (
 type Tombstone tombstone.Tombstone
 
 // NewTombstoneFromV2 wraps v2 Tombstone message to Tombstone.
+//
+// Nil tombstone.Tombstone converts to nil.
 func NewTombstoneFromV2(tV2 *tombstone.Tombstone) *Tombstone {
 	return (*Tombstone)(tV2)
 }

pkg/object/tombstone_test.go

@@ -5,6 +5,7 @@ import (
 	"crypto/sha256"
 	"testing"
 
+	"github.com/nspcc-dev/neofs-api-go/v2/tombstone"
 	"github.com/stretchr/testify/require"
 )
 
@@ -63,3 +64,11 @@ func TestTombstoneEncoding(t *testing.T) {
 		require.Equal(t, ts, ts2)
 	})
 }
+
+func TestNewTombstoneFromV2(t *testing.T) {
+	t.Run("from nil", func(t *testing.T) {
+		var x *tombstone.Tombstone
+
+		require.Nil(t, NewTombstoneFromV2(x))
+	})
+}