fix push info badge default value bug

First, I'm very thankful to you for this repository for giving me much convinience.
But I found that when I want to push a transmission message without apns notification, it always make a sound.
I looked into official python documents api, and found that this file igt_base_template.py:

    def getPushInfo(self):
        if self.pushInfo is None:
            self.pushInfo = gt_req_pb2.PushInfo()
            self.pushInfo.message = ""
            self.pushInfo.actionKey = ""
            self.pushInfo.sound = ""
            self.pushInfo.badge = "-1"
        return self.pushInfo

I edit it and it works. no sound any more.
I also compare the params['clientData"'] in http post request, and it is the same.
I wish that what I edit is just right. So may you take a look at it?