[#302] pkg/token: Convert nil BearerToken to nil message

Document that `BearerToken.ToV2` method return `nil`
when called on `nil`. Write corresponding unit tests.

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

pkg/token/bearer.go

@@ -23,7 +23,14 @@ type BearerToken struct {
 	token acl.BearerToken
 }
 
-func (b BearerToken) ToV2() *acl.BearerToken {
+// ToV2 converts BearerToken to v2 BearerToken message.
+//
+// Nil BearerToken converts to nil.
+func (b *BearerToken) ToV2() *acl.BearerToken {
+	if b == nil {
+		return nil
+	}
+
 	return &b.token
 }
 
@@ -100,6 +107,7 @@ func NewBearerToken() *BearerToken {
 	return b
 }
 
+// ToV2 converts BearerToken to v2 BearerToken message.
 func NewBearerTokenFromV2(v2 *acl.BearerToken) *BearerToken {
 	if v2 == nil {
 		v2 = new(acl.BearerToken)

pkg/token/bearer_test.go

@@ -55,3 +55,11 @@ func TestFilterEncoding(t *testing.T) {
 		require.Equal(t, f, d2)
 	})
 }
+
+func TestBearerToken_ToV2(t *testing.T) {
+	t.Run("nil", func(t *testing.T) {
+		var x *token.BearerToken
+
+		require.Nil(t, x.ToV2())
+	})
+}