GitHub Actions - empty env secrets

Update

While the original answer below does still apply to public repositories, there are a couple of new updates that may help for some use cases.

• If your repository is private, you can now enable workflows from forks.

• If your repository is public, there is a new pull_request_target event that is not subject to any token restrictions.

Original Answer

The reason you are experiencing this behaviour is because the Invite workflow is being triggered by a pull request from a forked repository.

With the exception of GITHUB_TOKEN, secrets are not passed to the runner when a workflow is triggered from a forked repository.

When this happens, the actor of the workflow is the user that opened the pull request. If that user doesn't have write access to your repository then they cannot use secrets (other than GITHUB_TOKEN).

Anyone with write access to a repository can create, read, and use secrets.

ref: https://help.github.com/en/actions/automating-your-workflow-with-github-actions/creating-and-using-encrypted-secrets#using-encrypted-secrets-in-a-workflow

If you run this step in your workflow you will see that it has nothing to do with your action. The TEST_SECRET secret won't be available in the workflow either.

      - name: Test
        env:
          TEST_GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
          TEST_SECRET: ${{ secrets.TEST_SECRET }}
        run: |
          echo ${#TEST_GITHUB_TOKEN}
          echo ${#TEST_SECRET}

Checking the event data in the GitHub context you'll see that actor is the user that forked the repository and opened the pull request.

      - name: Dump GitHub context
        env:
          GITHUB_CONTEXT: ${{ toJson(github) }}
        run: echo "$GITHUB_CONTEXT"

This is a different but related issue answered by a GitHub staff member where it's explained that these limitations on forked repositories exist to "prevent malicious actors from using actions to poison upstream or downstream repos."