don't send empty execute_result messages

if there's nothing to display, don't send the message.

IPython/kernel/zmq/displayhook.py

@@ -1,73 +1,74 b''
 """Replacements for sys.displayhook that publish over ZMQ."""
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 import sys
 
 from IPython.core.displayhook import DisplayHook
 from IPython.kernel.inprocess.socket import SocketABC
 from IPython.utils.jsonutil import encode_images
 from IPython.utils.py3compat import builtin_mod
 from IPython.utils.traitlets import Instance, Dict
 from .session import extract_header, Session
 
 class ZMQDisplayHook(object):
     """A simple displayhook that publishes the object's repr over a ZeroMQ
     socket."""
     topic=b'execute_result'
 
     def __init__(self, session, pub_socket):
         self.session = session
         self.pub_socket = pub_socket
         self.parent_header = {}
 
     def __call__(self, obj):
         if obj is None:
             return
 
         builtin_mod._ = obj
         sys.stdout.flush()
         sys.stderr.flush()
         msg = self.session.send(self.pub_socket, u'execute_result', {u'data':repr(obj)},
                                parent=self.parent_header, ident=self.topic)
 
     def set_parent(self, parent):
         self.parent_header = extract_header(parent)
 
 
 class ZMQShellDisplayHook(DisplayHook):
     """A displayhook subclass that publishes data using ZeroMQ. This is intended
     to work with an InteractiveShell instance. It sends a dict of different
     representations of the object."""
     topic=None
 
     session = Instance(Session)
     pub_socket = Instance(SocketABC)
     parent_header = Dict({})
 
     def set_parent(self, parent):
         """Set the parent for outbound messages."""
         self.parent_header = extract_header(parent)
 
     def start_displayhook(self):
         self.msg = self.session.msg(u'execute_result', {
             'data': {},
             'metadata': {},
         }, parent=self.parent_header)
 
     def write_output_prompt(self):
         """Write the output prompt."""
         self.msg['content']['execution_count'] = self.prompt_count
 
     def write_format_data(self, format_dict, md_dict=None):
         self.msg['content']['data'] = encode_images(format_dict)
         self.msg['content']['metadata'] = md_dict
 
     def finish_displayhook(self):
         """Finish up all displayhook activities."""
         sys.stdout.flush()
         sys.stderr.flush()
+        if self.msg['content']['data']:
         self.session.send(self.pub_socket, self.msg, ident=self.topic)
         self.msg = None