fix undefined method `empty?` exception for Fixnum zbx messages

Fix for zabbix output plugin while using with metrics filter

For example, 
```
metrics {
    meter => [ "http.%{[response]}" ]
}
```
generates Fixnum values
```
"http.200.count": 2274,
...
"http.302.count": 3188,
...
"http.404.count": 25,
```