Fix tests
Fix tests

getSentinelMember now does return a member (with just the userid) when there is no corresponding member yet.
With lazy loading it's perfectly possible the member is not available, and null breaks continuation in the timeline.

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