If not specified, a default Workstations service agent SA is returned

[modular-magician]

Closes #7411

According to the API spec:

Email address of the service account that will be used on VM instances used to support this config. This service account must have permission to pull the specified container image. If not set, VMs will run without a service account, in which case the image must be publicly accessible.

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Release Note Template for Downstream PRs (will be copied)

workstations:  fixed if `service_account` in  `google_workstations_workstation_config` isn't set, a default is set.

Derived from GoogleCloudPlatform/magic-modules#7428