add log if push token is empty when registering at nc server

Signed-off-by: Marcel Hibbe <dev@mhibbe.de>

app/src/main/java/com/nextcloud/talk/utils/PushUtils.java

@@ -302,6 +302,8 @@ public class PushUtils {
                     }
                 }
             }
+        } else {
+            Log.e(TAG, "push token was empty when trying to register at nextcloud server");
         }
     }