Fix ambiguous boolean evaluation in bert.py (#13236)

Signed-off-by: Xue, Zhan <[email protected]>

Author: zxue2

python/llm/src/ipex_llm/transformers/models/bert.py

@@ -119,7 +119,7 @@ def encoder_forward(
     output_hidden_states: Optional[bool] = False,
     return_dict: Optional[bool] = True,
 ):
-    if attention_mask and not attention_mask.any():
+    if attention_mask is not None and not attention_mask.any():
         attention_mask = None
     return BertEncoder.forward(
         self=self,