fix: handle Codeberg returning null entry in requested_reviewers

Codeberg can return null as part of requested_reviewers (presumably
because that user has been deleted). Handle that case, and also for
assignees since we're at it.

test/support/mock/codeberg-data.ts

@@ -87,7 +87,9 @@ export const CB_MERGED_PR_FIXTURE = {
       "received_events_url": "https://codeberg.org/api/v1/users/gh-user/received_events",
       "type": "User",
       "site_admin": false
-    }
+    },
+    // Sometimes Codeberg returns null as part of requested_reviewers.
+    null,
   ],
   "requested_teams": [