EventTimeStats.merge doesn't handle "zero.merge(zero)" correctly

Right now, EventTimeStats.merge doesn't handle "zero.merge(zero)". This will make "avg" become "NaN". And whatever gets merged with the result of "zero.merge(zero)", "avg" will still be "NaN". Then finally, "NaN".toLong will return "0" and the user will see the following incorrect report:

"eventTime" : {
    "avg" : "1970-01-01T00:00:00.000Z",
    "max" : "2019-01-31T12:57:00.000Z",
    "min" : "2019-01-30T18:44:04.000Z",
    "watermark" : "1970-01-01T00:00:00.000Z"
  }