Pass `null` to Mongo.Collection when generated with the `--where "client"` flag

Currently, creating a collection on the client only (by passing `--where "client"`) still passes the collection name in lowercase as a parameter to `Mongo.Collection`.  This leads to "Meteor: insert failed: Method not found" errors as the collection is not defined on the server.  This patch corrects that by assuming that if the user has passed the `--where "client"` flag, it is supposed to be an unsynchronised/local collection.

NB1 - the file extensions don't populate correctly in my test, but I'm pretty sure this is just because I'm installing a local npm package globally at present; I haven't touched anything that should impact this, so I'm pretty sure that it will still work fine when installed from repo.
NB2 - this is my first ever pull request, so apologies if it's a bit messy...