Fixed a serialization issue

retribute_api/cache.py

@@ -49,7 +49,11 @@ class Redis(Backend):
             v = await r.get(key)
         except KeyError:
             raise self.NotFound(key)
-        return json.loads(v)
+        try:
+            return json.loads(v)
+        except TypeError:
+            # Null, empty string, etc.
+            return v
 
     async def set(self, key, value):
         r = await self.redis()