null values in json are not handled correctly

[zUnixorn]

when sending a json with either valid_for or max_uses set to null, the return code is 400 Bad Request and a serde error message is returned in the response. The 400 may be wanted although null fields could just be treated as not existing, but the error message should not be a serde parsing error

reproduce with:
http POST https://s.flamion.dev/custom link=do_not_care max_uses=null valid_for=null
or
curl --request POST --header "Content-Type: application/json" --data '{"link":"do_not_care","valid_for":null,"max_uses":null}' https://s.flamion.dev/custom