Enable datafusion.optimizer.filter_null_join_keys by default

### 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

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### Describe alternatives you've considered

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### Additional context

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Start by locating the `datafusion.optimizer.filter_null_join_keys` setting and the `FilterNullJoinKeys` optimizer rule. Verify how the default is defined and how optimizer behavior is tested; done means the rule is enabled by default and existing or added tests confirm the resulting join-key filtering behavior.

Written by the indexing model from the issue text.