It is possible to rename a collection to an empty string

Mongo does not allow the creation of collections with an empty string as the name, but it is possible to rename an existing collection to "".

To reproduce:

> db[""].save(

{x:2}

) // it is not possible to create a new collection with an empty string as the name
invalid ns: test.
> db.getCollection("").save(

{x:2}

)
invalid ns: test.
> db.test.save(

{x:2}

) // it is possible to rename an existing collection to an empty string
> db.test.renameCollection("")

{ "ok" : 1 }

> db[""].find()

{ "_id" : ObjectId("4ede683f5833a9b2c91c8dc0"), "x" : 2 }

>