Fix condition to fetch id token from API

[golemiso]

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

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[modular-magician]

Oops! It looks like you're using an unknown release-note type in your changelog entries:

• REPLACEME

Please only use the types listed in https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md.

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I've detected that you're a community contributor. @hao-nan-li, a repository maintainer, has been assigned to assist you and help review your changes.

❓ First time contributing? Click here for more details

---

Your assigned reviewer will help review your code by:

• Ensuring it's backwards compatible, covers common error cases, etc.
• Summarizing the change into a user-facing changelog note.
• Passes tests, either our "VCR" suite, a set of presubmit tests, or with manual test runs.

You can help make sure that review is quick by running local tests and ensuring they're passing in between each push you make to your PR's branch. Also, try to leave a comment with each push you make, as pushes generally don't generate emails.

If your reviewer doesn't get back to you within a week after your most recent change, please feel free to leave a comment on the issue asking them to take a look! In the absence of a dedicated review dashboard most maintainers manage their pending reviews through email, and those will sometimes get lost in their inbox.

---

[hao-nan-li]

/gcbrun

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 1 file changed, 5 insertions(+), 4 deletions(-))
Terraform Beta: Diff ( 1 file changed, 5 insertions(+), 4 deletions(-))
TF Conversion: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2810
Passed tests 2507
Skipped tests: 301
Affected tests: 2

Action taken

Found 2 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

TestAccComputeNetworkEndpoints_networkEndpointsBasic|TestAccComputeFirewallPolicyRule_multipleRules

Get to know how VCR tests work

[modular-magician]

Tests passed during RECORDING mode:
TestAccComputeNetworkEndpoints_networkEndpointsBasic[Debug log]

Tests failed during RECORDING mode:
TestAccComputeFirewallPolicyRule_multipleRules[Error message] [Debug log]

Please fix these to complete your PR
View the build log or the debug log for each test