We write a "fake" applyOps with ns set to tenantId_ here and here

For shard merge, we won't have a tenant id, so the namespace string will end up looking like _., which is invalid when multitenancy is enabled, which means application of txn entries will fail.

We can't simply skip setting nss for Shard Merge because entries require nss to be set: https://github.com/10gen/mongo/blob/d84272507b3957ce6289a6545e53cb25331a88d6/src/mongo/db/repl/oplog_entry.idl#L77-L80

We should be able to set nss to NamespaceString(boost::none, "") for shard merge. As part of this ticket, we should add/update tests for <16MB transactions