ES|QL: ReplaceMissingFieldWithNull could lead to ClassCastException

[luigidellaquila]

ReplaceMissingFieldWithNull local logical optimization rule replaces expressions with a NULL Literal when a field is not mapped on local node.

This is dangerous because a Literal could not match the type of the expressions that are being replaced (eg. if the command expects a NamedExpression like in this case), and it can trigger a ClassCastException.

With current feature set this optimization is safe, but we could miss the problem when we add new commands.

Notice that this optimization triggers a few other downstream optimizations that can be quite important, see LocalLogicalPlanOptimizerTests.testMissingField*

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[alex-spies]

IMO this is a bug, so I marked this accordingly. This piece of code performs a wild, unchecked cast without any safeguards; this is bound to break again in the future, especially since we plan to add new logical plan nodes (and evolve existing ones).