see fomantic/Fomantic-UI#2926 issue and related discussion

Request Content-Type is/can be used by a webserver to determine the request type [1].

• Array/form data should always send Content-Type. Data of value "empty array" must send the header the same way as non-empty array data. This is also behaviour of native html form /wo any fields [2].

• non-array data ("null"/"undefined"/"false") can/should (probably) continue to send no Content-Type header implicitly

the LoC/condition to fix: https://github.com/jquery/jquery/blob/3.7.1/src/ajax.js#L638

[1] example https://github.com/Nyholm/psr7-server/blob/1.0.2/src/ServerRequestCreator.php#L62
[2] <form action="/p.aspx" method="post"><input type="submit"></form>