MatrixEvent.getRoomId() can return undefined

Nobody has claimed this yet.

I've lost some hours trusting the TypeScript annotation.

The return type should say `string|undefined`.

https://github.com/matrix-org/matrix-js-sdk/blob/67b8da719f6548be32c633cc28afecdf3dbabc5e/src/models/event.ts#L350-L356

btw. I didn't get this on a `m.presence` but on a `m.bad.encrypted`:

```
MatrixEvent {
_events: [Object: null prototype] {
'Event.decrypted': [ [Function: forSource], [Function] ],
'Event.replaced': [Function: forSource]
},
_eventsCount: 2,
_maxListeners: undefined,
event: {
content: {
algorithm: 'm.olm.v1.curve25519-aes-sha2',
sender_key: '8LgWTZ5yFIawz80ylkAl1Mc9BX1oUZGAAAl3z0E7lXM',
ciphertext: [Object]
},
type: 'm.room.encrypted',
sender: '@jaller94:localhost'
},
pushActions: { notify: true, tweaks: { highlight: false } },
_replacingEvent: null,
_localRedactionEvent: null,
_isCancelled: false,
clearEvent: {
type: 'm.room.message',
content: {
msgtype: 'm.bad.encrypted',
body: '** Unable to decrypt: Not included in recipients **'
}
},
senderCurve25519Key: null,
claimedEd25519Key: null,
forwardingCurve25519KeyChain: [],
untrusted: false,
_decryptionPromise: null,
retryDecryption: false,
txnId: null,
thread: null,
localTimestamp: NaN,
sender: null,
target: null,
status: null,
error: null,
forwardLooking: true,
verificationRequest: null,
reEmitter: ReEmitter { target: [Circular *1] },
[Symbol(kCapture)]: false
}
```

Start at src/models/event.ts around lines 350-356 and inspect MatrixEvent.getRoomId(). Confirm how the method behaves for events without a room ID, including the m.bad.encrypted example. The work is done when the TypeScript return annotation accurately allows an absent value.

Written by the indexing model from the issue text.