server/user_token: empty notes after strip will be saved as nulls

server/szurubooru/func/user_tokens.py

@@ -140,6 +140,7 @@ def update_user_token_expiration_time(
 def update_user_token_note(user_token: model.UserToken, note: str) -> None:
     assert user_token
     note = note.strip() if note is not None else ''
+    note = None if len(note) == 0 else note
     if util.value_exceeds_column_size(note, model.UserToken.note):
         raise InvalidNoteError('Note is too long.')
     user_token.note = note