Fixes a bug introduced in
https://github.com/matrix-org/matrix-react-sdk/pull/426.
Particularly when we are showing search results, we may not recognise the
sender of an event; attempting to create a MemberAvatar for it will lead to
null-reference errors.
Also a bit of untangling of the logic of needsSenderProfile. Since
https://github.com/matrix-org/matrix-react-sdk/pull/422,
EventTileType.needsSenderProfile was only being called on MessageEvents, and
therefore only returned true. It's a shame to see all this logic going into
EventTile rather than the individual EventTileTypes, but since it's there,
let's not leave the unused logic lying around in the EventTileType
implementations.
