Allow Transcript to handle missing 'confidence' fields

[daspecster]

When max_alternitives > 1, the confidence field is omitted entirely. So Transcript needs to set confidence to None.

{
 "results": [
  {
   "alternatives": [
    {
     "transcript": "hello thank you for using Google Cloud platform",
     "confidence": 0.95869493
    },
    {
     "transcript": "thank you for using Google Cloud platform"
    },
    {
     "transcript": "hello thank you for using Google Cloud platforms"
    }
   ]
  }
 ]
}