Don't crash when getting unknown collections

Fix #422.

radicale/__init__.py

@@ -409,6 +409,9 @@ class Application:
         else:
             # Get whole collection
             answer = collection.serialize()
+            if answer is None:
+                return client.NOT_FOUND, {}, None
+            else:
                 etag = collection.etag
 
         if answer:

radicale/storage.py

@@ -543,6 +543,8 @@ class Collection(BaseCollection):
         return time.strftime("%a, %d %b %Y %H:%M:%S GMT", time.gmtime(last))
 
     def serialize(self):
+        if not os.path.exists(self._filesystem_path):
+            return None
         items = []
         for href in os.listdir(self._filesystem_path):
             path = os.path.join(self._filesystem_path, href)