[bugfix] fix margin (#5817)

Author: Jintao-Huang

swift/llm/template/base.py

@@ -369,7 +369,7 @@ def _rlhf_encode(self, inputs: TemplateInputs) -> Dict[str, Any]:
             data = locals()[f'{prefix}_encoded']
             for k, v in data.items():
                 encoded[f'{prefix}_{k}'] = v
-        if margin:
+        if margin is not None:
             encoded['margin'] = float(margin)
         return encoded