Enable datafusion.optimizer.filter_null_join_keys by default

[Dandandan]

Is your feature request related to a problem or challenge?

It is better to have FilterNullJoinKeys enabled by default, as pushing a filter down below a join is generally faster than the overhead saved by not having to execute the null filter.
Besides that, in many cases the filter can be pushed down further to the scan or might enable other optimizations (now or future ones).

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

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