Room List: Scroll to top of the list when active room is not in the list (#29650)

* Scroll to top when active room is not in list

So that when filters are applied and the active room is not in the list
anymore, the list is scrolled to the top.

* Write test

src/components/views/rooms/RoomListPanel/RoomList.tsx

@@ -43,7 +43,7 @@ export function RoomList({ vm: { rooms, activeIndex } }: RoomListProps): JSX.Ele
                         rowHeight={48}
                         height={height}
                         width={width}
-                        scrollToIndex={activeIndex}
+                        scrollToIndex={activeIndex ?? 0}
                     />
                 )}
             </AutoSizer>