make value checking for span_indices explicit

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/tokenization_utils_base.py

@@ -521,7 +521,7 @@ def token_to_chars(self, batch_or_token_index: int, token_index: Optional[int] =
             token_index = batch_or_token_index
         span_indices = self._encodings[batch_index].token_to_chars(token_index)
 
-        return CharSpan(*span_indices) if span_indices else None
+        return CharSpan(*span_indices) if span_indices is not None else None
 
     def char_to_token(
         self, batch_or_char_index: int, char_index: Optional[int] = None, sequence_index: int = 0