Invite failed to warn me that the invited user might not exist

Moved from https://github.com/matrix-org/synapse/issues/12735. The original reporter was @felixx9.

### Steps to reproduce

- User A wants to invite User B, but accidentially invites User C (which does not have an account / does not exist yet)
- User A does write a lot of confidential stuff into the newly created room
- User C (which might be a completely different person than User B) creates his account
- he sees the invitation & accepts it

### Outcome

#### What did you expect?

- I would expect, that
- I see this warning (but I didn't): https://github.com/vector-im/element-web/issues/7137 within Client (e.g. Element)._
- 
- the room is encrypted
- if not possible due to missing public key (?) of User C: Do not create the room
- if created an encrypted: User C is not allowed to join, (unless invited again after account creation.)

#### What happened instead?

- User C can read the whole history, which was meant for User B
- the room is not encrpyted (even though the servers default setting says: encrypt all rooms)

### Operating system

_No response_

### Application version

Element Desktop 1.10.8

### How did you install the app?

_No response_

### Homeserver

matrix.yatrix.org, Synapse > 1.58.1

### Will you send logs?

No