Fix condition to fetch id token from API

[modular-magician]

I use GCE metadata server to authenticate and want to get id token.

  data "google_service_account_id_token" "oidc" {
    target_audience = "https://foo.bar/"
  }

The condition if creds.JSON == nil prevents me from passing the line idtoken.NewTokenSource() and returns incomprehensible error message below:

Error: error calling iamcredentials.GenerateIdToken: googleapi: got HTTP response code 404 with body: 

  with data.google_service_account_id_token.oidc,
  on provider.tf line 6, in data "google_service_account_id_token" "oidc":
   6: data "google_service_account_id_token" "oidc" {

I think this can be fixed by that the API is called only if target_service_account is provided.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read Write release notes before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

resourcemanager: fixed handling of `google_service_account_id_token` when authenticated with GCE metadata credentials

Derived from GoogleCloudPlatform/magic-modules#8195