Fix un-marking rooms as DM rooms

src/Rooms.js

@@ -107,11 +107,13 @@ export function setDMRoom(roomId, userId) {
     }
 
     // now add it, if it's not already there
+    if (userId) {
         const roomList = dmRoomMap[userId] || [];
         if (roomList.indexOf(roomId) == -1) {
             roomList.push(roomId);
         }
         dmRoomMap[userId] = roomList;
+    }
 
 
     return MatrixClientPeg.get().setAccountData('m.direct', dmRoomMap);