pruning empty fields from a message

gistfile1.txt

input {
  generator {
    type => "stdin"
    lines => [ "this is an %{empty.match} in a string" ]
    count => 1
  }
}

filter {
  mutate {
    type    => "stdin"
    gsub => [ "@message", "%{[^}]*}", "" ]
  }
}

output {
  stdout {
    debug        => true
    debug_format => "json"
  }
}