fix empty messages when sending custom payload to socketio

[santteegt]

trafficstars

Proposed changes:

• Fixes socketio channel to avoid sending empty messages when a bot utterance contains a custom payload

Fixes #7344

Status (please check what you already did):

• [ ] added some tests for the functionality
• [ ] updated the documentation
• [ ] updated the changelog (please check changelog for instructions)
• [X] reformat files using black (please check Readme for instructions)

[CLAassistant]

All committers have signed the CLA.

[sara-tagger]

Thanks for submitting a pull request 🚀 @kalkbrennerei will take a look at it as soon as possible ✨

[tmbo]

@kalkbrennerei do you have time to take a look at this PR?

[m-vdb]

@santteegt are you up for wrapping up this PR?

[santteegt]

Hi @m-vdb,

Yeah, however, I'm not sure if I should write unit tests for the changes I made as there are no existing unit tests for socketio channel. I was waiting for some direction here https://github.com/RasaHQ/rasa/pull/7373#discussion_r547562142

[m-vdb]

I don't know much about this part of the codebase, but I think it'd be great if we could have a test that covers the change you're introducing, even if there's none today. If it requires a reasonable amount of work (that I can't tell for certain)

[stale[bot]]

This PR has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.