Fixes 'undefined' default SIP number.

app.js

 
 function callSipButtonClicked()
 {
+    var defaultNumber
+        = config.defaultSipNumber ? config.defaultSipNumber : '';
+
     messageHandler.openTwoButtonDialog(null,
         '<h2>Enter SIP number</h2>' +
             '<input id="sipNumber" type="text"' +
-            ' value="' + config.defaultSipNumber + '" autofocus>',
+            ' value="' + defaultNumber + '" autofocus>',
         false,
         "Dial",
         function (e, v, m, f) {

config.js

     useNicks: false,
     bosh: '//jitsi-meet.example.com/http-bind', // FIXME: use xep-0156 for that
     clientNode: 'http://jitsi.org/jitsimeet', // The name of client node advertised in XEP-0115 'c' stanza
+    //defaultSipNumber: '', // Default SIP number
     desktopSharing: 'ext', // Desktop sharing method. Can be set to 'ext', 'webrtc' or false to disable.
     chromeExtensionId: 'diibjkoicjeejcmhdnailmkgecihlobk', // Id of desktop streamer Chrome extension
     desktopSharingSources: ['screen', 'window'],