fix: Crash in argument parsing when using Openrouter (#5667)

## Why are these changes needed?
See issue for a bug description.
The problem was that a lot of openrouter models return `""` as
`tool_call.arguments`, which caused `json.loads` to fail
## Related issue number
#5666
---------

Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>

python/packages/autogen-agentchat/src/autogen_agentchat/agents/_assistant_agent.py

@@ -566,7 +566,7 @@ async def _execute_tool_call(
             tool = next((t for t in self._tools + self._handoff_tools if t.name == tool_call.name), None)
             if tool is None:
                 raise ValueError(f"The tool '{tool_call.name}' is not available.")
-            arguments = json.loads(tool_call.arguments)
+            arguments: Dict[str, Any] = json.loads(tool_call.arguments) if tool_call.arguments else {}
             result = await tool.run_json(arguments, cancellation_token)
             result_as_str = tool.return_value_as_string(result)
             return (tool_call, FunctionExecutionResult(content=result_as_str, call_id=tool_call.id, is_error=False))