fix: fix if message.content is none

chat_template.jinja

@@ -18,6 +18,8 @@
                 {{- item }}
             {%- endif -%}
         {%- endfor -%}
+    {%- elif content is none -%}
+        {{- '' }}
     {%- else -%}
         {{- content }}
     {%- endif -%}