Ensure roomId are not rendered in the UI.

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Use RoomAvatar data fallback everywhere, to not use roomId (`!` char) for the avatar initial, but rather `#`.

libraries/matrix/test/src/main/kotlin/io/element/android/libraries/matrix/test/room/RoomSummaryFixture.kt

@@ -63,7 +63,7 @@ fun aRoomSummaryFilled(
 
 fun aRoomSummaryDetails(
     roomId: RoomId = A_ROOM_ID,
-    name: String = A_ROOM_NAME,
+    name: String? = A_ROOM_NAME,
     isDirect: Boolean = false,
     avatarUrl: String? = null,
     lastMessage: RoomMessage? = aRoomMessage(),