Non-empty message appears empty

### Description

The content of a non-empty text message appears as empty.

Viewing the source of the message shows its content, and other clients show the message properly (tested with Element Android, Fractal, and FluffyChat).


### Steps to reproduce

Unknown, but here are a few properties of the message that may have caused this:
- It's an edit
- Its `formatted_body` (both the original and the edit) uses `'` as an apostrophe character, in two locations
- It was a Discord message bridged by `matrix-appservice-discord` (though this shouldn't be any fault of the bridge)

### Version information

- **Platform**: web
- **Browser**: Firefox 86.0
- **OS**: Windows 10 & Fedora 33 (Workstation Edition)
- **URL**: app.element.io & self-hosted client on versions 1.7.22 & 1.7.23