If we have no displayName then always show the mxid

[ara4n]

The LL code introduces a fallback where we show the userId, which is really confusing for debugging things like #7332. Let's fail hard and always show an mxid if we are missing a displayName, given Riot should always set a displayName during signup.

[bwindels]

We are already showing the mxid (called userId in the RoomMember code, hence the confusion), so closing this.