Fix unreachable code mypy warnings in elasticsearch provider

[yunchipang]

related: #53395

original warning:
providers/elasticsearch/src/airflow/providers/elasticsearch/log/es_task_handler.py:511: error: Statement is unreachable [unreachable]

this was caused by:

if self.handler is None:
    self.closed = True
    return

because self.handler was not allowed to be None in the type hint, mypy considered the if condition always false, marking self.closed = True as unreachable.

fix: updated the type of self.handler to include None and initialized it with None.

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[jscheffl]

I think this change is reasonable. But another expert review who knows elastic would be good.