Extra margin above and below big emoji when contained in paragraph

### Description

When big emoji in chats is enabled, extra spacing is shown above and below big emoji when it's contained within a paragraph.

```
"formatted_body": "

🙂

",
```

There is no extra spacing when the message does not contain just an emoji.

### Steps to reproduce

These messages are delivered by a bridge, I don't know how to force the emoji to be in `

` tags using riot-web.

### Version information

- both web and desktop

For the web app:

- **Browser**: Chrome, Firefox
- **OS**: Arch Linux
- **URL**: riot.im/develop

For the desktop app:

- **OS**: Arch Linux
- **Version**: 1.6.2 (probably older too)

Reproduce the issue with a chat message whose formatted body is

🙂

, using the web or desktop app described in the report. Trace how big emoji are rendered and styled, then verify that a paragraph containing only one big emoji has no extra spacing above or below while mixed-content messages retain their expected layout.

Written by the indexing model from the issue text.