allow no add_prefix

This PR allows config like below:
```
<match full.input>
  interval 10
  add_prefix 
  remove_prefix full
</match>

<match input>
  ...
</match>
```

But I have one matter.
Because `add_prefix` has default parameter, we need specify `add_prefix` alone to make blank.
This behavior looks odd. Do you have any idea?