Fix a couple of PHP notices when the 'ua' and 'callback' parameters are not specfied:
Fix a couple of PHP notices when the 'ua' and 'callback' parameters are not specfied:

Notice: Undefined index: ua in /home/dwc/src/MIT-Mobile-Web/web/api/index.php on line 38

Notice: Undefined index: callback in /home/dwc/src/MIT-Mobile-Web/web/api/index.php on line 55

welps/MIT-Mobile-Web,welps/MIT-Mobile-Web,welps/MIT-Mobile-Web