grok overwrite not working for empty strings about logstash-filter-grok HOT 2 OPEN

input {
  generator { count => 1 message => "<14>2015-02-11T17:49:29Z logspout dev_ziservice_1[1]: ASDF"}
  generator { count => 1 message => "<14>2015-02-11T17:49:29Z logspout dev_ziservice_1[1]: "}
  generator { count => 1 message => "<14>2015-02-11T17:49:29Z logspout dev_ziservice_1[1]:"}
}
filter {
  grok {
    match => ["message", "<%{NUMBER}>%{TIMESTAMP_ISO8601:syslogTimestamp} %{SYSLOGHOST} %{DATA:container_name}(?:\[%{POSINT}\])?:%{SPACE}?%{GREEDYDATA:message}"]
    overwrite => [ "message" ]
  }
}
output {
  stdout { codec => rubydebug }
}