GridFS.list() sometimes includes None

This is surprising. If there's a file in a GridFS collection with no filename, GridFS.list doesn't include None. But after calling get_last_version or get_version, list will include None:

>>> import pymongo

>>> import gridfs

>>> client = pymongo.MongoClient()

>>> fs = gridfs.GridFS(client.db)

>>> fs.put(b'hello')

ObjectId('52882cf5ca1ce99eed9a0524')

>>> fs.list()

[]

>>> fs.get_last_version()

<gridfs.grid_file.GridOut object at 0x1012cb4d0>

>>> fs.list()

[None]

This is because get_last_version or get_version ensure an index on (filename, uploadDate). list calls distinct on filename, and the distinct command behaves differently depending on whether it uses an index or not: SERVER-11747.