[CALCITE-5390] RelDecorrelator throws NullPointerException

[dssysolyatin]

Ticket: [CALCITE-5390]

My time for calcite contributions is limited. I will have time to continue this work only next month. If anyone wishes to take over or continue this task in the meantime, please feel free to do so.
I currently have a draft solution, but it has a few problems that need resolving:

• Solution logic throws a NullPointerException in cases where a correlation variable is used but has not been properly introduced. This specifically happens in join plans where a subquery exists in the ON clause, but not in the WHERE clause. (See example: [CALCITE-7286]). Also in this case of [CALCITE-7286] LogicalJoin should be replaced by LogicalCorrelate
• Solution depends on a proposal to introduce a new RelNode.copy method with a 3rd parameter to replace the variablesSet [CALCITE-7280]. As suggested by Stamatis in a comment on this task, this functionality should probably be implemented using the RelBuilder instead.
• Better test coverage
• At the moment, code only compiles with Guava version 32.0 or higher because of buildKeepingLast.

PR contains two commits. The first commit relates to [CALCITE-7280] (introducing copy method) and second contains the logic for the main fix itself. Feel free to add any comment you wish, especially if you know a better solution.

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