Fallback for empty AccessToken

[ali-sattari]

Fallback to extra fields provided if AccessToken is empty (case witt GCP IAP)

Fixes #480

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[ali-sattari]

@googlebot I signed it!

[manugarg]

Thanks for this PR, @ali-sattari. Please take a look at my comment.

[manugarg]

Looks good now. Thanks.

I'll start the merge process as per:
https://github.com/google/cloudprober/blob/master/CONTRIBUTING.md#source-of-truth-sot-and-commit-process

[manugarg]

Commited through #483: f40ed7e.

Thank you once again!