Bug: issue_comment trigger can run CLA workflow on non-PR issues

[AndrasZiegler]

Describe the bug
issue_comment trigger
To Reproduce

1. Open a normal GitHub Issue (not a PR).
2. Add the signature comment to the issue.
3. Observe that the workflow is triggered for that issue comment.

Expected behavior
The CLA workflow should only run for:

• pull_request_target events, or
• issue_comment events where the comment is on a pull request (github.event.issue.pull_request exists).

Non-PR issue comments should not trigger CLA processing.

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.