Fix handling of empty values for required list parameters

The command ```ElasticLoadBalancing#set_load_balancer_policies_of_listener``` has a required parameter policy_names which is defined as a list of policy names (list of strings). The documentation for this option states:

List of policies to be associated with the listener. Currently this list can have at most one policy. If the list is empty, the current policy is removed from the listener.

If you supply an empty array for the value an exception will be thrown:

Aws::ElasticLoadBalancing::Errors::ValidationError (1 validation error detected: Value null at 'policyNames' failed to satisfy constraint: Member must not be null)

This change allows you to pass an empty array.