Fix Activity Input check (#13)

Signed-off-by: Deepanshu Agarwal <deepanshu.agarwal1984@gmail.com>

Author: DeepanshuA

CHANGELOG.md

@@ -4,6 +4,10 @@
 
 - Add gRPC metadata option ([#16](https://github.com/microsoft/durabletask-python/pull/16)) - contributed by [@DeepanshuA](https://github.com/DeepanshuA)
 
+### Fixed
+
+- Fix zero argument values sent to activities as None ([#13](https://github.com/microsoft/durabletask-python/pull/13)) - contributed by [@DeepanshuA](https://github.com/DeepanshuA)
+
 ### Changes
 
 - Removed Python 3.7 support due to EOL ([#14](https://github.com/microsoft/durabletask-python/pull/14)) - contributed by [@berndverst](https://github.com/berndverst)

durabletask/worker.py

@@ -370,7 +370,7 @@ def call_activity(self, activity: Union[task.Activity[TInput, TOutput], str], *,
                       input: Union[TInput, None] = None) -> task.Task[TOutput]:
         id = self.next_sequence_number()
         name = activity if isinstance(activity, str) else task.get_name(activity)
-        encoded_input = shared.to_json(input) if input else None
+        encoded_input = shared.to_json(input) if input is not None else None
         action = ph.new_schedule_task_action(id, name, encoded_input)
         self._pending_actions[id] = action
 
@@ -386,7 +386,7 @@ def call_sub_orchestrator(self, orchestrator: task.Orchestrator[TInput, TOutput]
         if instance_id is None:
             # Create a deteministic instance ID based on the parent instance ID
             instance_id = f"{self.instance_id}:{id:04x}"
-        encoded_input = shared.to_json(input) if input else None
+        encoded_input = shared.to_json(input) if input is not None else None
         action = ph.new_create_sub_orchestration_action(id, name, instance_id, encoded_input)
         self._pending_actions[id] = action