Parsing XML with self-closing tags

If I use logstash to parse XML with an self-closing tag (e.g. <foo />), it turns it into an empty object instead of an empty string. For example:

"parsed" => {
    "foo" => [
        [0] {}

How can I add a conditional to only call add_field if it's a string, not an empty object? If I do this:

if [parsed][foo][0] {
    mutate {
        add_field => {
            "foo" => "%{[parsed][foo][0]}"

the field "foo" gets the value "{}". I also tried checking if [parsed][foo][0] != "{}", but that didn't catch it.

I'm not sure is it correct way or not, but i've solved this problem with removing empty (self closing) xml tags with mutate filter and gsub config. My code is below;

mutate { gsub => [ "message", "<(\s)?\w+(\s)?/>", "" ] }