Made nic_type default to a string (and expand as a string)

[modular-magician]

Related to GoogleCloudPlatform/terraform-validator#195

Nic type expansion was causing a panic in terraform validator because a nil value couldn't be converted to an interface. We don't actually need to expand nic_type, because it will be a string.

When I generate tgc & the provider locally and force tfv to use the local versions with replace directives, tfv is able to read plan json that contains null values for nic_type without any issues.

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Derived from GoogleCloudPlatform/magic-modules#4598