Remove dummy NODE_AUTH_TOKEN export

[gowridurgad]

Description:
The action previously exported a dummy NODE_AUTH_TOKEN value (XXXXX-XXXXX-XXXXX-XXXXX) when no token was provided. While this didn't break OIDC flows, it could corrupt the user's .npmrc by injecting a non-functional token value into the environment, potentially causing confusing behavior during OIDC publish. This PR removes the dummy fallback and only exports NODE_AUTH_TOKEN when the user has explicitly set it.

Related issue:
#1440

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[Copilot]

Pull request overview

Removes the previous dummy fallback value for NODE_AUTH_TOKEN during registry auth setup, so the action only exports NODE_AUTH_TOKEN when it is explicitly present in the environment—preventing unintended token injection that can interfere with npm OIDC publishing flows.

Changes:

• Stop exporting a placeholder NODE_AUTH_TOKEN when the variable is not set.
• Preserve behavior for explicitly set tokens, including an explicit empty-string token.
• Update the compiled dist output and add unit tests covering the new export behavior.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
src/authutil.ts	Gate NODE_AUTH_TOKEN export behind an explicit presence check in process.env.
dist/setup/index.js	Regenerated compiled output reflecting the new export behavior.
__tests__/authutil.test.ts	Adds coverage for “not set” and “set to empty string” NODE_AUTH_TOKEN cases.

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