🐛 [#5012] Fix handling of response after post request in JSON dump

If there is no content (in case of an HTTP 204 response for example), converting the response to json results in an error, so we just return an empty string

src/openforms/registrations/contrib/json_dump/plugin.py

@@ -99,7 +99,9 @@ def register_submission(
             result = client.post(path, json=data)
             result.raise_for_status()
 
-        return {"api_response": result.json()}
+        result_json = result.json() if result.content else ""
+
+        return {"api_response": result_json}
 
     def check_config(self) -> None:
         # Config checks are not really relevant for this plugin right now