fix: don't send just " " in body, send an empty string

[mickaelmagniez]

Fix #10560

Change the default body sent from " " (space character) to empty body.

Because some backends (for example akka-http) failed to parse it as json.

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@cchafer

[wing328]

cc @clasnake (2017/07), @jimschubert (2017/09) ❤️, @shijinkui (2018/01), @ramzimaalej (2018/03), @chameleon82 (2020/03), @Bouillie (2020/04)

[chameleon82]

@wing328 sorry, don't have permissions to approve this PR.
PR Looks good to me

[wing328]

@chameleon82 thanks for the quick review.

@mickaelmagniez thanks for the PR.