Don't try to collapse zero events with a group
Fixes https://github.com/vector-im/riot-web/issues/12423
When events are redacted they fail to make it into the Grouper because the `shouldAddEvent` check blocks them from entering. However, the grouper expects that when `getTiles()` is called that there's events to group and dutifully grabs some context from the array. Because JavaScript is the least helpful language, `myArray[-1]` returns `undefined` or `null` and thus you get `cannot read 'sender' of undefined`.
Regressed in https://github.com/matrix-org/matrix-react-sdk/pull/4059
