`null` converted to `[]`

JSON `null` values are not round-tripped. For example, this `savejson(loadjson('{"myNullValue": null}'))` results in:
```
ans =

    '{
     	"myNullValue":[]
     }
     '
```

The problem appears to be here: https://github.com/fangq/jsonlab/blob/cf573262eb12c1d1dc3668913c1f902e3452f566/loadjson.m#L560
