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".

*.pyc
venv/
build/lib
build/messages/_cache
dist/
piecrust.egg-info/
piecrust/__version__.py