Don't break if mixpanel settings are not loaded.

Previously, if there were no Meteor settings, the mixpanel package wouldn't find
the mixpanel token in Meteor.settings.public.mixpanel. Then, the first time we
try to track a metric, the app would explode causing routes not to render. So,
this time, if mixpanel is not initialized, just don't record the metric instead
of failing completely.