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".

language: python
python:
  - "3.4"
install: "pip install -r requirements.txt"
script: py.test