fix failure when no backend specified (#8)

[neomantra]

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:

hashicorp/hil#50 (comment)

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

Contributed according to the Google CLA.

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[neomantra]

I already signed it as a corporate entity (Neomantra Corp) and am the Point of Contact. Thanks!

[googlebot]

CLAs look good, thanks!

[danisla]

Sorry about the long delay on this one, LGTM. Thanks

[hyww]

This merge seems conflict with 45c9c34

Mod behavior of element() is used in 45c9c34 , thus appending extra elements to the original list should be avoided.