Set a default value for envs in Cloud Run Job

[modular-magician]

What?

• Make a similar change to Add default value to env.value to fix the plan diffs in cloud run services GoogleCloudPlatform/magic-modules#13363 but for the google_cloud_run_v2_job resource.
  • Fixes Cloud Run env variables reordering causing resource to update in-place terraform-provider-google#7467
  • Fixes Cloud Run it's recreating all the secret environment variables when adding/removing a secret env terraform-provider-google#10634

Why?

• Currently, when a new env variable is added to the env set, the env set is re-ordered which results in a large diff, see Cloud Run env variables reordering causing resource to update in-place terraform-provider-google#7467 for an example. The cause for this seems to be that in the Terraform state the env value is "" when using a value_source (i.e. a reference to a google_secret_manager_secret), but in the plan the value is nil (as it has not been set), which then triggers all of the envs to have changed.

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cloudrunv2: fixed the diffs for unchanged `template.template.containers.env` in `google_cloud_run_v2_job` resource

Derived from GoogleCloudPlatform/magic-modules#13487