fix execute a_generate_reply with function_call: None (#3250)

Co-authored-by: Jack Gerrits <jackgerrits@users.noreply.github.com>

autogen/agentchat/conversable_agent.py

@@ -1658,8 +1658,8 @@ class ConversableAgent(LLMAgent):
         if messages is None:
             messages = self._oai_messages[sender]
         message = messages[-1]
-        if "function_call" in message:
-            func_call = message["function_call"]
+        func_call = message.get("function_call")
+        if func_call:
             func_name = func_call.get("name", "")
             func = self._function_map.get(func_name, None)
             if func and inspect.iscoroutinefunction(func):

test/agentchat/test_function_and_tool_calling.py

@@ -33,6 +33,7 @@ async def _a_tool_func_error(arg1: str, arg2: str) -> str:
 _tool_use_message_1 = {
     "role": "assistant",
     "content": None,
+    "function_call": None,
     "tool_calls": [
         {
             "id": "1",
@@ -56,6 +57,7 @@ _tool_use_message_1 = {
 _tool_use_message_1_bad_json = {
     "role": "assistant",
     "content": None,
+    "function_call": None,
     "tool_calls": [
         {
             "id": "1",