fix: Uncaught TypeError: Cannot convert undefined or null to object in livedata_connection.js

[mjcctech]

We are using https://github.com/wojtkowiak/meteor-custom-protocol and some messages have null value for msg. Unfortunately code checking this case has a bug and end up with error message in browser console

livedata_connection.js:1780 Uncaught TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at Connection.onMessage (livedata_connection.js:1780:20)
    ...

[CLAassistant]

All committers have signed the CLA.

[nachocodoner]

For me this is ok. I have seen this error on the past and hopefully this covers the fix.

However, since this change impacts the core, I'd appreciate another experienced set of eyes, like @denihs, to review this. Tests are passing, but it's crucial to ensure we haven't missed any scenario with this change.