add message.content check to prevent UndefinedError

If an assistant message lacks content (e.g., tool-call-only messages), accessing message.content will cause a Jinja2 UndefinedError

chat_template.jinja

@@ -42,7 +42,7 @@
                         {%- endif %}
                         
                 {%- if add_generation_prompt and not (message.reasoning_content is defined and message.reasoning_content is not none) %}
-                    {%- if '</think>' in message.content %}
+                    {%- if message.content is defined and '</think>' in message.content %}
                         {%- set content = message.content.split('</think>'.strip())[-1].lstrip('\n') %}
                     {%- endif %}
                 {%- endif %}