set invitation email as optional

proto/invitations/invitations.proto

@@ -11,7 +11,7 @@ package content.invitations;
 
 message Invitation {
   string id = 1;
-  string email = 2;
+  optional string email = 2;
   string org_id = 3;
   string space_id = 4;
   string owner_id = 5;