Update _assistant_agent.py

## Description
When using Claude models (or other LLM providers) with tool calling, the `tool_call.arguments` 
field may be an empty string `""` instead of `"{}"` for tools that don't require parameters.

This causes a `JSONDecodeError[ in ](file://core/mt_llm_client.py#90#24)AssistantAgent._execute_tool_call` method, even though 
the tool call is valid and should be executed with empty arguments.

## Current Behavior
```python
# In _assistant_agent.py line 1547
arguments = json.loads(tool_call.arguments)  # Raises JSONDecodeError for ""
```

When `tool_call.arguments = ""`, this raises:

Author: linger118927

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

@@ -1544,7 +1544,11 @@ async def _execute_tool_call(
         """Execute a single tool call and return the result."""
         # Load the arguments from the tool call.
         try:
-            arguments = json.loads(tool_call.arguments)
+            # Handle empty string as empty dict to support tools with no parameters
+            if not tool_call.arguments or tool_call.arguments.strip() == "":
+                arguments = {}
+            else:
+                arguments = json.loads(tool_call.arguments)
         except json.JSONDecodeError as e:
             return (
                 tool_call,