Ensure roomId are not rendered in the UI.

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

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/roomlist/RoomSummary.kt

@@ -37,7 +37,7 @@ sealed interface RoomSummary {
 
 data class RoomSummaryDetails(
     val roomId: RoomId,
-    val name: String,
+    val name: String?,
     val canonicalAlias: RoomAlias?,
     val isDirect: Boolean,
     val avatarUrl: String?,