Handle a TTL of 0 correctly

[sebnow]

When a TTL override of 0 is set, the override is overriden with the
policy due to using the || operator, which considers 0 to be false.

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[apollo-cla]

@sebnow: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[abernix]

Makes sense to me. Thanks!