Document that Future.result() may return None (#884)

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

rclpy/rclpy/task.py

@@ -88,7 +88,7 @@ def result(self):
 
         :raises: Exception if one was set during the task.
 
-        :return: The result set by the task
+        :return: The result set by the task, or None if no result was set.
         """
         if self._exception:
             raise self.exception()