Fix `pad_token` check condition (#25685)

fix
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

src/transformers/tokenization_utils_base.py

@@ -2503,7 +2503,7 @@ class PreTrainedTokenizerBase(SpecialTokensMixin, PushToHubMixin):
                     max_length = self.model_max_length
 
         # Test if we have a padding token
-        if padding_strategy != PaddingStrategy.DO_NOT_PAD and (not self.pad_token or self.pad_token_id < 0):
+        if padding_strategy != PaddingStrategy.DO_NOT_PAD and (self.pad_token is None or self.pad_token_id < 0):
             raise ValueError(
                 "Asking to pad but the tokenizer does not have a padding token. "
                 "Please select a token to use as `pad_token` `(tokenizer.pad_token = tokenizer.eos_token e.g.)` "