fix failure when no backend specified (#8)
fix failure when no backend specified (#8)

This change sets the `ports` list to [""] when `backend_params` is empty.

Ideally this would be done with an expression like:
```
"${length(var.backend_params) == 0 ? "" : element(split(",", element(var.backend_params, count.index)), 2)}"
```

But because of a limitation with HCL, it cannot be done and we've resorted
to the trick described in this issue:

https://github.com/hashicorp/hil/issues/50#issuecomment-320078816

The pipe `|` delimiter is used because it is not valid to use in GCP names.

Contributed according to the Google CLA.

terraform-google-modules/terraform-google-lb-http