Only run Auto Assign PR workflow if PR is not merged (#17888)

It appears that currently the `.github/workflows/auto-assign.yml` workflow runs on every PR commit even when an assignee has already been set (e.g. https://github.com/rapidsai/cudf/actions/runs/13062684572/job/36449057944) and on merge on the current branch (e.g. https://github.com/rapidsai/cudf/actions/runs/13072179599/job/36476031430).

I think in theory we would only want to run this workflow once upon PR opening (?), but I added a condition to run this workflow if a PR does not have an assignee (name). cc @Matt711

Authors:
  - Matthew Roeschke (https://github.com/mroeschke)
  - Vyas Ramasubramani (https://github.com/vyasr)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)
  - Matthew Murray (https://github.com/Matt711)

URL: #17888

.github/workflows/auto-assign.yml

@@ -10,6 +10,7 @@ on:
 jobs:
   add_assignees:
     runs-on: ubuntu-latest
+    if: ${{ ! github.event.pull_request.merged }}
     steps:
       - uses: actions-ecosystem/action-add-assignees@v1
         with: