[Issue]: Fix type and default value of the code_execution_config parameter in UserProxyAgent

[davorrunje]

Describe the issue

UserProxyAgent declares code_execution_config to be an optional parameter and sets its default value for to None

autogen/autogen/agentchat/user_proxy_agent.py

Line 33 in 8f6590e

code_execution_config: Optional[Union[Dict, Literal[False]]] = None,

, while ConversableAgent deprecated use of None

autogen/autogen/agentchat/conversable_agent.py

Line 83 in 8f6590e

code_execution_config: Union[Dict, Literal[False]] = False,

Steps to reproduce

Instantiating a user proxy results in a deprecating warning. E.g. by running the following test:

pytest test/agentchat/test_agent_setup_with_use_docker_settings.py -k test_agent_setup_with_env_variable_false_and_docker_running

we get the following warning

============================================== warnings summary ===============================================
test/agentchat/test_agent_setup_with_use_docker_settings.py::test_agent_setup_with_env_variable_false_and_docker_running
  /workspaces/autogen/autogen/agentchat/user_proxy_agent.py:83: UserWarning: Using None to signal a default code_execution_config is deprecated. Use {} to use default or False to disable code execution.
    super().__init__(

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html```

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