Only auto-assign PRs from forks if they don't already have assignees

[martinflorian-da]

Otherwise it seems like we're added back as assignees whenever a PR is edited.

I prefer to unassign myself from PRs that are already shepherded by someone, so they stop showing up in my gh status and other overviews.

Pull Request Checklist

Cluster Testing

• If a cluster test is required, comment /cluster_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.
• If a hard-migration test is required (from the latest release), comment /hdm_test on this PR to request it, and ping someone with access to the DA-internal system to approve it.

PR Guidelines

• Include any change that might be observable by our partners or affect their deployment in the release notes.
• Specify fixed issues with Fixes #n, and mention issues worked on using #n
• Include a screenshot for frontend-related PRs - see README or use your favorite screenshot tool

Merge Guidelines

• Make the git commit message look sensible when squash-merging on GitHub (most likely: just copy your PR description).

[moritzkiefer-da]

I do like the behavior where you're assigned to everything but this probably makes more sense

[martinflorian-da]

FYI @isegall-da @ray-roestenburg-da