Fix assertion error when workflow initialization fails

The serialization context changes introduced an incorrect 'assert workflow' statement that would fail when workflow initialization fails (e.g., when the workflow class isn't registered). This caused tests to hang with an unhandled AssertionError.

The workflow variable can legitimately be None when initialization fails, and we only need it when applying a payload codec. Changed to check for workflow existence as part of the condition for applying the codec.

Author: dandavison

temporalio/worker/_workflow.py

@@ -354,10 +354,9 @@ async def _handle_activation(
                 )
 
         completion.run_id = act.run_id
-        assert workflow
 
         # Encode completion
-        if data_converter.payload_codec:
+        if data_converter.payload_codec and workflow:
             payload_codec = _CommandAwarePayloadCodec(workflow.instance)
             try:
                 await temporalio.bridge.worker.encode_completion(