Q:

I need my module to return either a list of items if input is a non-empty list or an empty list

My module takes a possibly-empty-list as input, and if that list is non-empty, creates some resources and returns a specific attribute that I need outside of the module, like so:
variable contexts {
    type = "list"
}

resource "pagerduty_service" "p1" {
    count = "${length(var.contexts)}"
    name                    = "p1-${element(var.contexts, count.index)}"
    description             = "p1-${element(var.contexts, count.index)}"
    auto_resolve_timeout    = 14400
    acknowledgement_timeout = 1800
    escalation_policy       = "${pagerduty_escalation_policy.p1.id}"
    alert_creation          = "create_alerts_and_incidents"
    incident_urgency_rule {
        type    = "constant"
        urgency = "high"
    }
}

data "pagerduty_vendor" "cloudwatch" {
    name = "Cloudwatch"
}

resource "pagerduty_service_integration" "p1_cloudwatch" {
    count   = "${length(var.contexts)}"
    name    = "Amazon Cloudwatch"
    vendor  = "${data.pagerduty_vendor.cloudwatch.id}"
    service = "${element(pagerduty_service.p1.*.id, count.index)}"
}

output "integration_keys" {
    value = "${pagerduty_service_integration.*.integration_keys}"
}

The trouble I am having is that when this module is run first with a non-empty list, thus creating the resources, it works fine. If I run it again, it fails with this exception:
* module.pagerduty.output.integration_keys: Resource 'pagerduty_service_integration.possibly_empty_resource_list' does not have attribute 'integration_key' for variable 'pagerduty_service_integration.possibly_empty_resource_list.*.integration_key'

I can't figure out a nice way to have this output return an empty list if the possibly_empty_resource_list is empty.
Any ideas?
EDIT:
I tried performing a ternary check on the output, but for some reason, using a list is not supported so this won't work however I hope it illustrates what I am trying to do:
"${length(var.contexts) > 0 ? pagerduty_service_integration.*.integration_keys : list()}"

Solution:
output "instance_id" { 
  value = "${element(concat(aws_instance.example.*.id, list("")), 0)}"
}

A:

There's a section at the very bottom of the terraform upgrade to 0.11 guide here: https://www.terraform.io/upgrade-guides/0-11.html that shows what I use for counted resources
ex:
output "instance_id" { value = "${element(concat(aws_instance.example.*.id, list("")), 0)}" }
(moved over from a comment)

