Fix the None return case

src/python/grpcio/grpc/_cython/_cygrpc/aio/server.pyx.pxi

@@ -264,15 +264,18 @@ async def _finish_handler_with_unary_response(RPCState rpc_state,
     rpc_state.raise_for_termination()
 
     # Serializes the response message
-    cdef bytes response_raw = serialize(
-        response_serializer,
-        response_message,
-    )
+    cdef bytes response_raw
+    cdef SendMessageOperation send_message_op
+    if rpc_state.status_code == StatusCode.ok:
+        response_raw = serialize(
+            response_serializer,
+            response_message,
+        )
+        send_message_op = SendMessageOperation(response_raw, rpc_state.get_write_flag())
 
     # Assembles the batch operations
     cdef tuple finish_ops
     finish_ops = (
-        SendMessageOperation(response_raw, rpc_state.get_write_flag()),
         SendStatusFromServerOperation(
             rpc_state.trailing_metadata,
             rpc_state.status_code,
@@ -280,6 +283,8 @@ async def _finish_handler_with_unary_response(RPCState rpc_state,
             _EMPTY_FLAGS,
         ),
     )
+    if rpc_state.status_code == StatusCode.ok:
+        finish_ops += (send_message_op,)
     if not rpc_state.metadata_sent:
         finish_ops = prepend_send_initial_metadata_op(
             finish_ops,