Don't ignore the cached NULLs in bulk createOrUpdate for Mongo

The bulk createOrUpdate operation needs to have old versions of all updated documents. If some documents are not in the cache, they are requested from the Mongo. However, if the document key is cached with the NULL value (which means it doesn't exists), the createOrUpdate ignores such information and tries to load the document from Mongo anyway.

We shouldn't ignore these NULLs but assume that keys cached with the NULL value indicate an non-existing document.

If the document exists (because the cache became outdated), the sendBulkUpdate() method will detect it.