fixes #10 by catching exception

This fixes #10 but it still generates a warning:

```
record_reformer: NoMethodError undefined method `[]' for nil:NilClass /Users/marshall/code/random/fluent-plugin-record-reformer/lib/fluent/plugin/out_record_reformer.rb:190:in `block in expand'
```

I'm not sure what the best way to handle this is, this is an improvement because it does not stop it from processing the message, which I think is better than the current behavior in which it will stop processing. 

Since we have several different message formats we do not want to stop processing. 
Well, okay, I re-considered. 

Returning `nil` instead of `str` is fine, but you should not emit a message with `nil` tag, and should return `nil` explicitly without using the return value of `log.warn`.