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>

routers/api/packages/auth.go

@@ -32,14 +32,14 @@ func (a *Auth) Verify(req *http.Request, w http.ResponseWriter, store auth.DataS
 	}
 
 	if packageMeta == nil || packageMeta.UserID == 0 {
-		return nil, nil
+		return nil, nil //nolint:nilnil // the auth method is not applicable
 	}
 
 	var u *user_model.User
 	switch packageMeta.UserID {
 	case user_model.GhostUserID:
 		if !a.AllowGhostUser {
-			return nil, nil
+			return nil, nil //nolint:nilnil // the auth method is not applicable
 		}
 		u = user_model.NewGhostUser()
 	case user_model.ActionsUserID: