"Must pass start_service to Class[Foo::Service]

init.pp

class foo(
  $start_service = undef
) {
  class{'::foo::service':
    start_service => $start_service,
  }
}

service.pp

class foo::service(
  $start_service,
) {

  service{'foo':
    ensure => $start_service
  }
  
}

service.pp - alternate

class foo::service(
  $start_service,
) {

  $real_start_service = $start_service ? {
    'UNSET' => undef,
    default => $start_service,
  }
  
  service{'foo':
    ensure => $real_start_service
  }
  
}