Fix detekt issue:

Use `?.let {}` instead of if/else with a null block when checking for nullable values [UseLet]

features/roomlist/impl/src/main/kotlin/io/element/android/features/roomlist/impl/model/RoomListRoomSummaryProvider.kt

@@ -49,7 +49,7 @@ internal fun aRoomListRoomSummary(
     name: String = "Room name",
     hasUnread: Boolean = false,
     lastMessage: String? = "Last message",
-    timestamp: String? = if (lastMessage != null) "88:88" else null,
+    timestamp: String? = lastMessage?.let { "88:88" },
     isPlaceholder: Boolean = false,
     notificationMode: RoomNotificationMode? = null,
     hasRoomCall: Boolean = false,