Fix upsert on insert new document with defined id

After this commit: https://github.com/doctrine/mongodb-odm/commit/8251bb9bf943e6abfdb7c347fa1ad6383c795c88
when I trying to insert document with defined id, I get an error: ContextErrorException: Notice: Undefined index: $set in /opt/local/www/realty/vendor/doctrine/mongodb-odm/lib/Doctrine/ODM/MongoDB/Persisters/DocumentPersister.php line 290

I have wrote details about problem here: https://github.com/doctrine/mongodb-odm/commit/8251bb9bf943e6abfdb7c347fa1ad6383c795c88#commitcomment-5114932

Sorry, I don't know doctrine well, just try to fix this problem. Maybe should be another solution. Thanks.