didn't look at the `readOptionalStreamable` function previously, i thought it will not read/write anything if the `TranslogStats` is null.
After look at the `readOptionalStreamable` and `writeOptionalStreamable` function, it seems it doing the same thing as 
```java
//read
if (in.readBoolean()) {
  segments = SegmentsStats.readSegmentsStats(in);
}

//write
        if (segments == null) {
            out.writeBoolean(false);
        } else {
            out.writeBoolean(true);
            segments.writeTo(out);
        }

```
etc. 

should the read/write for all stats using `readOptionalStreamable` and `writeOptionalStreamable` for consistency?