Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

services/issue/issue.go

@@ -229,7 +229,7 @@ func AddAssigneeIfNotAssigned(ctx context.Context, issue *issues_model.Issue, do
 	}
 	if isAssigned {
 		// nothing to do
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil because the user is already assigned
 	}
 
 	valid, err := access_model.CanBeAssigned(ctx, assignee, issue.Repo, issue.IsPull)