PROTON-2838: [Python] Treat empty list custom disposition as None

Custom dispositions would be more consistently represented as empty
lists rather than null: The only current use for descriptors is to
describe lists.

python/proton/_delivery.py

@@ -188,7 +188,8 @@ def data(self) -> Optional[List[int]]:
         if self.local:
             return self._data
         else:
-            return dat2obj(pn_disposition_data(self._impl))
+            r = dat2obj(pn_disposition_data(self._impl))
+            return r if r != [] else None
 
     @data.setter
     def data(self, obj: List[int]) -> None: