coalescelist does not allow all lists to be empty

[nicgordon]

There was a change in the behaviour of the coalescelist function in terraform 0.12 (see this issue) that means that if you pass it all empty lists it will throw an error. This poses a problem for this module on lines like this if you are relying on default values.

[hanswesterbeek]

Ran into this issue when using https://github.com/cloudposse/terraform-aws-ecs-web-app