HTTP 500 if you try to set membership to "join" for a federated account

This issue has been migrated from [#11598](https://github.com/matrix-org/synapse/issues/11598).

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### Description

If you try to set a custom `m.room.member` state event, a lot of things are caught and result in an HTTP 400. Membership value is invalid? HTTP 400. State key is no Matrix ID? HTTP 400.

However, if both are valid and you set membership to `"join"` for someone other than you, you get an HTTP 500.

### Steps to reproduce

- Create a room (any room where you're a moderator).
- Send an `m.room.member` state event with any Matrix-ID-looking value other than your Matrix ID. The content must be `{"membership": "join"}`.

I expect this to be an HTTP 400, telling me that I cannot join others without their interaction. I can set their membership to "invite", "leave" and "ban" but not "invite" (or "knock").

### Version information

- **Homeserver**:

If not matrix.org: vector.modular.im

- **Version**: 1.48.0, maybe?

- **Install method**: EMS

- **Platform**: EMS