Python invoker throws exception when the function returns just data without attributes

[andrew-su]

Bug Report

Running a function that expects cloudevents where after processing returns only the body will cause it to throw an exception.

Expected Behavior

No errors + proper attributes are returned.

Steps to Reproduce the Bug

1. Define a function:

   from typing import Any
   
   def main(data: Any, attributes: dict):
       # Your function implementation goes here
       return data
   

2. Run this function with the Python invoker.
3. Send a CE via curl to function.