fix: preserve null_equals_null flag in eliminate_cross_join rule

[waynexia]

Which issue does this PR close?

• Closes #.

Rationale for this change

I found that a join plan created using join_detailed method with null_equals_null set to true will be rewritten to a join plan with null_equals_null set to false. This is because optimizer rule eliminate_cross_join will assemble a Join plan and hardcode a false value. This patch fixes that rule and now it will respect the null_equals_null flag from input plan.

A little more background: In a normal SQL model, JOIN and many other places usually take null != null. But I want to leverage JOIN to "concat" two tables' columns in a PromQL scenario, where I treat null as just an identical vacancy, that requires null = null.

What changes are included in this PR?

A bug fix to eliminate_cross_join plan

Are these changes tested?

I wrote a new unit test case to cover this case.

Are there any user-facing changes?

No.

[alamb]

Thank you @waynexia -- this makes sense to me

I was thinking that we might be able to extract a common pattern for join rewriting here and avoid a similar bug (losing the value of null_equals_null) in other optimizer passes that rewrite joins. However, I don't have any good suggestions on how to improve things