fix $setOnInsert with empty obj cause mongodb 2.6 complain

With the new mongodb 2.6, `$setOnInsert` with empty object will cause this error:

```
findAndModifyFailed failed: {
	"errmsg" : "exception: '$setOnInsert' is empty. You must specify a field like so: {$mod: {<field>: ...}}",
	"code" : 9,
	"ok" : 0
}
```

So I use it only when its content is not empty.