mautrix-discord cannot relay messages

*This issue was originally created by [**@Elara6331**](https://github.com/Elara6331) at .*

### Background information

- **Dendrite version or git SHA**: 0.13.6+e9deb52
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: Yes
- **`go version`**: N/A
- **Client used (if applicable)**: Element

### Description

- **What** is the problem: The mautrix-discord bridge fails to relay messages from discord to my matrix homeserver. Dendrite is returning HTTP 500.
- **Who** is affected: Users of the bridge on my homeserver
- **How** is this bug manifesting: Dendrite returns Internal Server Error. The following error message appears in mautrix-discord logs:
```
ERR Failed to send part of message to Matrix error="failed to ensure joined: failed to POST /_matrix/client/v3/rooms/!Sk5rRcx0XrZ87BkT:genzedong.xyz/join: HTTP 500" action="discord message create" attachment_id= author_id= channel_id= channel_receiver= message_id= message_type=0 part_index=0 room_id=!Sk5rRcx0XrZ87BkT:genzedong.xyz
```
Dendrite doesn't log any errors, it only logs the following message:
```
level=info msg="User requested to room join" room_id="!Sk5rRcx0XrZ87BkT:genzedong.xyz" servers="[]" user_id="@discord_:genzedong.xyz"
```
- **When** did this first appear: After setting up the bridge

### Steps to reproduce

- Set up mautrix-discord bridge
- Try sending message on the Discord side

### Additional Information

#2696 had a similar issue