[#302] pkg/container: Convert nil Attributes to nil message

Make `Attributes.ToV2` method to return `nil`
when called on `nil`. Make `NewAttributesFromV2`
function to return `nil` when called on `nil`.
Write corresponding unit tests.

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

pkg/container/attribute.go

@@ -50,6 +50,10 @@ func (a *Attribute) ToV2() *container.Attribute {
 }
 
 func NewAttributesFromV2(v []*container.Attribute) Attributes {
+	if v == nil {
+		return nil
+	}
+
 	attrs := make(Attributes, 0, len(v))
 	for i := range v {
 		attrs = append(attrs, NewAttributeFromV2(v[i]))
@@ -59,6 +63,10 @@ func NewAttributesFromV2(v []*container.Attribute) Attributes {
 }
 
 func (a Attributes) ToV2() []*container.Attribute {
+	if a == nil {
+		return nil
+	}
+
 	attrs := make([]*container.Attribute, 0, len(a))
 	for i := range a {
 		attrs = append(attrs, a[i].ToV2())

pkg/container/attribute_test.go

@@ -61,6 +61,14 @@ func TestAttribute(t *testing.T) {
 }
 
 func TestAttributes(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x container.Attributes
+
+		require.Nil(t, x.ToV2())
+
+		require.Nil(t, container.NewAttributesFromV2(nil))
+	})
+
 	var (
 		keys = []string{"key1", "key2", "key3"}
 		vals = []string{"val1", "val2", "val3"}