Handle empty permissions for packaged apps (bug 1039067)

https://bugzilla.mozilla.org/show_bug.cgi?id=1039067

Handle the case where there aren't any permissions in the manifest which the app-validator helpfully represents as `False` instead of `[]`.
I agree. I filed a [bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1040364) for this but this code is going to be backed out later so unless it's going to stay I'll leave it as-is.