Fix the AssertionError about count

```
Traceback (most recent call last):
File "/home/user/app/app.py", line 29, in <module>
soccer_news_tool = SoccerNewsTool()
File "/usr/local/lib/python3.10/site-packages/smolagents/tools.py", line 68, in new_init
self.validate_arguments()
File "/usr/local/lib/python3.10/site-packages/smolagents/tools.py", line 184, in validate_arguments
assert "nullable" in value, (
AssertionError: Nullable argument 'count' in function signature should have key 'nullable' set to True in inputs.
```

tools.py

@@ -67,6 +67,7 @@ class SoccerNewsTool(Tool):
         "count": {
             "type": "integer",
             "description": "How many headlines to return (1-20, default 5).",
+            "nullable": True,
             "default": 5,
         }
     }