PhabricatorRepository::parseRepositoryServicePath() can return null per https://we.phorge.it/source/phorge/browse/master/src/applications/repository/storage/PhabricatorRepository.php;123831b53fb7572cba11e9c990dcb9d247614890$635, thus make the code in DiffusionServeController:getRequestDirectoryPath() at least not crash when handling null and instead fall back to serving a "403 This repository is read-only over HTTP."
There is probably a cleaner approach which I happily leave to any future person willing to figure it out.
Closes T15944