## What changes were proposed in this pull request?
Right now, EventTimeStats.merge doesn't handle `zero.merge(zero)` correctly. This will make `avg` become `NaN`. And whatever gets merged with the result of `zero.merge(zero)`, `avg` will still be `NaN`. Then finally, we call `NaN.toLong` and get `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"
}
```
This issue was reported by liancheng .
This PR fixes the above issue.
## How was this patch tested?
The new unit tests.
Closes#23718 from zsxwing/merge-zero.
Authored-by: Shixiong Zhu <zsxwing@gmail.com>
Signed-off-by: Shixiong Zhu <zsxwing@gmail.com>
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