authorize: populate issuer even when policy is nil

[kenjenkins]

Summary

After the recent changes to internal policy evaluation, the /.pomerium/jwt endpoint was returning empty strings for the JWT aud and iss claims.

Related issues

Fixes #4210

User Explanation

Fix an issue where the /.pomerium/jwt endpoint returned a JWT missing the audience and issuer claims.

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[coveralls]

Coverage: 63.589% (+0.03%) from 63.561% when pulling 68736f8 on kenjenkins/jwt-claims into d315e68 on main.

[backport-actions-token]

The backport to 0-21-0 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-0-21-0 0-21-0
# Navigate to the new working tree
cd .worktrees/backport-0-21-0
# Create a new branch
git switch --create backport-4211-to-0-21-0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 6df4fba83236c8401fa487c0575453b0fd759d63
# Push it to GitHub
git push --set-upstream origin backport-4211-to-0-21-0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-0-21-0

Then, create a pull request where the base branch is 0-21-0 and the compare/head branch is backport-4211-to-0-21-0.