Fix error on absence of "Mongo" symbol
Fix bug in 0.9.1 where reactive dict throws an error if the mongo package is not present.

Summary:
Add a check for Mongo being undefined.

This is the only package that has a weak dependency on mongo.

Test Plan: Ran test-packages, but I don't think test-packages caught this in the first place... maybe we need to test weak dependencies somehow.

Reviewers: avital

Reviewed By: avital

CC: emily, nim

Differential Revision: https://phabricator.meteor.com/D810