Allow empty arrays as values in grammar

Allow following config not to fail with parse error

<pre>
filter {
  grok {
    tags                => [ "apache-accesslog" ]
    patterns_dir        => [ "/opt/logstash/patterns" ]
    match               => [ "clientip", "%{IP}" ]
    break_on_match      => true
    named_captures_only => true
    tag_on_failure      => [ ]
  }
}
</pre>