Fix Discord not rendering payload_json

It seems Discord no longer renders the `payload_json` if we pass an
empty string filename; we have to completely exclude the filename from
the multipart/form-data header.

discord_integration/log.py

@@ -50,7 +50,7 @@ def message(self, message: Dict[str, Any], full_message: Optional[str] = None) -
             files.append(('full_message.txt', full_message, 'text/plain'))
 
         data = {
-            'payload_json': ('', json.dumps(message), 'application/json'),
+            'payload_json': (None, json.dumps(message), 'application/json'),
         }
         for i, file in enumerate(files):
             data['files[{}]'.format(i)] = file