[#288] pkg/eacl: Convert nil eACL record to nil API v2 message

Make `Record.ToV2` method to return `nil` when called on `nil`. Write
corresponding unit test.

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

pkg/acl/eacl/record.go

@@ -128,7 +128,13 @@ func (r *Record) AddObjectOwnerIDFilter(m Match, id *owner.ID) {
 // TODO: add remaining filters after neofs-api#72
 
 // ToV2 converts Record to v2 acl.EACLRecord message.
+//
+// Nil Record converts to nil.
 func (r *Record) ToV2() *v2acl.Record {
+	if r == nil {
+		return nil
+	}
+
 	targets := make([]*v2acl.Target, 0, len(r.targets))
 	for _, target := range r.targets {
 		targets = append(targets, target.ToV2())

pkg/acl/eacl/record_test.go

@@ -119,3 +119,11 @@ func TestRecordEncoding(t *testing.T) {
 		require.Equal(t, r, r2)
 	})
 }
+
+func TestRecord_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *Record
+
+		require.Nil(t, x.ToV2())
+	})
+}