Fix websocket reconnection when a guest joins the conversation

When the standalone signaling server is used and a participant joins a
conversation a "join" signaling message is received. The code assumed
that the event always contains a "user" attribute, but that attribute is
empty for guests. As the "user" attribute was used unconditionally this
caused an exception, and as the exception happened when handling a
websocket message it propagated until it caused a reconnection of the
websocket.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

app/src/main/java/com/nextcloud/talk/webrtc/MagicWebSocketInstance.java

@@ -247,7 +247,10 @@ public class MagicWebSocketInstance extends WebSocketListener {
                                             HashMap<String, Object> userMap = (HashMap<String, Object>) internalHashMap.get("user");
                                             participant = new Participant();
                                             participant.setUserId((String) internalHashMap.get("userid"));
-                                            participant.setDisplayName((String) userMap.get("displayname"));
+                                            if (userMap != null) {
+                                                // There is no "user" attribute for guest participants.
+                                                participant.setDisplayName((String) userMap.get("displayname"));
+                                            }
                                             usersHashMap.put((String) internalHashMap.get("sessionid"), participant);
                                         }
                                     }