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/chef/auth.go

@@ -61,7 +61,7 @@ func (a *Auth) Verify(req *http.Request, w http.ResponseWriter, store auth.DataS
 		return nil, err
 	}
 	if u == nil {
-		return nil, nil
+		return nil, nil //nolint:nilnil // the auth method is not applicable
 	}
 
 	pub, err := getUserPublicKey(req.Context(), u)
@@ -88,7 +88,7 @@ func (a *Auth) Verify(req *http.Request, w http.ResponseWriter, store auth.DataS
 func getUserFromRequest(req *http.Request) (*user_model.User, error) {
 	username := req.Header.Get("X-Ops-Userid")
 	if username == "" {
-		return nil, nil
+		return nil, nil //nolint:nilnil // the auth method is not applicable
 	}
 
 	return user_model.GetUserByName(req.Context(), username)