Empty mixins break component selector

[XuluWarrior]

If an app includes an empty mixin, selecting a component using either the magnifying glass and clicking on the component name or clicking the View / Component in the View Tree causes a call stack in ember-inspector in propertiesForMixin(mixin) where it expects all mixins to have a mixins array.

Version 0.6.2 of ember-component-css includes such an empty mixin and so breaks ember-inspector for apps that use it.

You can see the behaviour when running https://github.com/XuluWarrior/ember-component-css-test

I have already raised webark/ember-component-css#274 but it seems that ember-inspector should be robust enough to deal with empty mixins.