serve: Fix a bug where empty route metadata is not the same as invalid route. Some routes can capture no metadata at all, like for example when `foo/%path:slug%` is capturing URL `foo`. The route is still valid, so it returns an empty dictionary. It's not the same as returning `None`, which means "no match".

[pytest]
norecursedirs = .* _darcs CVS .svn .git .hg *.egg-info dist build node_modules venv tmp* {args}