formatted_body is not used for nic.custom.confetti

### Steps to reproduce

1. Send a `/confetti` message with a custom emoji from nheko
2. The custom emoji is not displayed in Element, instead it shows the HTML code
3. Sending a custom emoji without `/confetti` works fine

confetti message

```json
{
"content": {
"body": "",
"format": "org.matrix.custom.html",
"formatted_body": "",
"msgtype": "nic.custom.confetti"
},
// […]
"type": "m.room.message",
// […]
}
```

message without confetti

```json
{
"content": {
"body": "",
"format": "org.matrix.custom.html",
"formatted_body": "",
"msgtype": "m.text"
},
// […]
"type": "m.room.message",
// […]
}
```

### Outcome

#### What did you expect?

The emoji is rendered like with normal messages.

#### What happened instead?


### Operating system

Linux

### Application version

Element Desktop 1.11.17 / 1.11.8

### How did you install the app?

[Void Linux repo](https://github.com/void-linux/void-packages/blob/master/srcpkgs/element-desktop/template) / [Gentoo repo](https://gitweb.gentoo.org/repo/gentoo.git/tree/net-im/element-desktop-bin/element-desktop-bin-1.11.8-r1.ebuild?id=6748725db4aec61990c9493cef340bb189cc28b2)

### Homeserver

Synapse 1.74.0

### Will you send logs?

No