fix tool call blank content (#2458)

src/backend/src/services/ai/utils/Messages.js

@@ -40,7 +40,7 @@ export const normalize_single_message = (message, params = {}) => {
                 });
             }
             delete message.tool_calls;
-        } else {
+        } else if ( !message.role === 'tool' ) {
             throw new Error('each message must have a \'content\' property');
         }
     }