Handle null userID
Handle null userID

The tests inject messages with no sender, so for now let's maintain
behaviour as it was before.

krombel/matrix-js-sdk,krombel/matrix-js-sdk,matrix-org/matrix-js-sdk,krombel/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk