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".

from textile import textile
from piecrust.formatting.base import Formatter


class TextileFormatter(Formatter):
    FORMAT_NAMES = ['textile', 'text']
    OUTPUT_FORMAT = 'html'

    def render(self, format_name, text):
        assert format_name in self.FORMAT_NAMES
        return textile(text)