Update network_ip to use empty string

[tpdownes]

Enables compatibility with

https://github.com/GoogleCloudPlatform/slurm-gcp/blob/5b883492c8b99d8da5e7cc39c06a9d15b5700c85/terraform/slurm_cluster/modules/_instance_template/main.tf#L147

I will submit a PR to that repo to make the code treat null and empty string identically.

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