Fix migration session id multiversion failure

Migration fails from 3.2 mongod to 3.0 mongod.

In a migration, a 3.2 donor mongod generates a migration session id, saves it, and then sends it to a 3.0 recipient mongod, which ignores it. The 3.0 recipient then sends requests to the 3.2 donor without a session id. The 3.2 donor rejects the empty session id requests because it doesn't match the id it generated...

If the 3.2 donor receives a request without a session id, ignore the match results.