[#288] pkg/eacl: Convert nil eACL filter to nil message

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

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

pkg/acl/eacl/filter.go

@@ -66,7 +66,13 @@ func (f Filter) From() FilterHeaderType {
 }
 
 // ToV2 converts Filter to v2 acl.EACLRecord.Filter message.
+//
+// Nil Filter converts to nil.
 func (f *Filter) ToV2() *v2acl.HeaderFilter {
+	if f == nil {
+		return nil
+	}
+
 	filter := new(v2acl.HeaderFilter)
 	filter.SetValue(f.value.String())
 	filter.SetKey(f.key.String())

pkg/acl/eacl/filter_test.go

@@ -59,3 +59,11 @@ func TestFilterEncoding(t *testing.T) {
 		require.Equal(t, f, d2)
 	})
 }
+
+func TestFilter_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *Filter
+
+		require.Nil(t, x.ToV2())
+	})
+}