Fix navbar rendering when navigation.stack is empty (bug 1088419)

https://bugzilla.mozilla.org/show_bug.cgi?id=1088419

It shouldn't happen, but thanks to New Relic we have evidence that it does. My guess is that there is a race condition somewhere causing the navbar to be rendered before navigation stack is built.

This adds a simple check to prevent raising a `TypeError` that completely breaks Marketplace for the affected users.