Support running AppRTC without a TURN ICE server

The "ice_server_url" key is always present in AppRTC server's JSON response but may be empty if no ICE server is wanted.

BUG=webrtc:7636

Review-Url: https://codereview.webrtc.org/2878673002
Cr-Commit-Position: refs/heads/master@{#18118}

webrtc/examples/androidapp/src/org/appspot/apprtc/RoomParametersFetcher.java

@@ -139,7 +139,7 @@ public class RoomParametersFetcher {
         }
       }
       // Request TURN servers.
-      if (!isTurnPresent && roomJson.has("ice_server_url")) {
+      if (!isTurnPresent && !roomJson.optString("ice_server_url").isEmpty()) {
         LinkedList<PeerConnection.IceServer> turnServers =
             requestTurnServers(roomJson.getString("ice_server_url"));
         for (PeerConnection.IceServer turnServer : turnServers) {