Fix type and default value of the code_execution_config parameter in UserProxyAgent (#1996)

* fix type and default value of the code_execution_config of UserProxAgent

* fix type and default value of the code_execution_config of UserProxAgent

* set default value of llm_config in UserProxyAgent to None

* fixed tests

* revert llm_config to False

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Co-authored-by: Chi Wang <wang.chi@microsoft.com>