Disable test-publish when credentials are not present

[michaelwheeler]

The test-publish job fails on PRs from dependabot forks (and probably all forks).

Notice: Attempting to perform trusted publishing exchange to retrieve a temporary short-lived API token for authentication against https://test.pypi.org/legacy/ due to __token__ username with no supplied password field
Error: Trusted publishing exchange failure: 
OpenID Connect token retrieval failed: GitHub: missing or insufficient OIDC token permissions, the ACTIONS_ID_TOKEN_REQUEST_TOKEN environment variable was unset

This generally indicates a workflow configuration error, such as insufficient
permissions. Make sure that your workflow has `id-token: write` configured
at the job level, e.g.:

permissions:
  id-token: write

Learn more at https://docs.github.com/en/actions/deployment/security-hardening-your-deployments/about-security-hardening-with-openid-connect#adding-permissions-settings.