Consider ComponentSpec non-default if group overrides are not allowed based on platform restrictions

Because an a GroupOverride with just an haproxy is removed in GroupOverride.reduce, we must change the logic of GroupOverride.empty? in the multi-platform case.

Initially we wanted to change ComponentSpec.is_default? so it returns false if the current spec is for a web proxy and application contains web frameworks that are of a different platform. That approach is not possible because application.cartridges is not populated at the time Application.update_requirements is called.