Markdown editor is broken for particular messages.

[haiodo]

Error msg:

index.es.js?36fa:1380 Uncaught (in promise) RangeError: Empty text nodes are not allowed
    at new TextNode (index.es.js?36fa:1380)
    at Schema.text (index.es.js?36fa:2375)
    at Function.fromJSON (index.es.js?36fa:1368)
    at Schema.nodeFromJSON (index.es.js?36fa:2389)
    at Array.map (<anonymous>)
    at Function.fromJSON (index.es.js?36fa:292)
    at Function.fromJSON (index.es.js?36fa:1370)
    at Schema.nodeFromJSON (index.es.js?36fa:2389)
    at Array.map (<anonymous>)
    at Function.fromJSON (index.es.js?36fa:292)

Steps to reproduce:

1. Copy a text https://ru.lipsum.com from and paste to chunter message.

2. Open message editing.