🐛 Fix setting contentType even tho there's no body

src/rest/request.ts

@@ -90,7 +90,7 @@ export class APIRequest {
       )
       form.append('payload_json', JSON.stringify(body))
       body = form
-    } else {
+    } else if (body !== undefined) {
       contentType = 'application/json'
       body = JSON.stringify(body)
     }