SQL: do not attempt to canonicalize InnerAggregate

[luigidellaquila]

InnerAggregate does not support replaceChildren(), so we should not try to canonicalize it either.

It's a very corner case, but it could happen (ref. ES-12727)

java.lang.UnsupportedOperationException: can't be rewritten at org.elasticsearch.xpack.ql.expression.function.aggregate.InnerAggregate.replaceChildren(InnerAggregate.java:51)
at org.elasticsearch.xpack.ql.expression.function.aggregate.InnerAggregate.replaceChildren(InnerAggregate.java:18)
at org.elasticsearch.xpack.ql.tree.Node.replaceChildrenSameSize(Node.java:231)
at org.elasticsearch.xpack.ql.expression.Expression.canonicalize(Expression.java:165)
at org.elasticsearch.xpack.ql.expression.Expression.canonical(Expression.java:151)
at org.elasticsearch.xpack.ql.expression.Expression.semanticHash(Expression.java:173)
at org.elasticsearch.xpack.ql.expression.predicate.BinaryOperator.canonicalize(BinaryOperator.java:55)
at org.elasticsearch.xpack.ql.expression.Expression.canonical(Expression.java:151)
at org.elasticsearch.xpack.ql.expression.Expression.semanticEquals(Expression.java:169)
at org.elasticsearch.xpack.sql.expression.predicate.conditional.NullIf.foldable(NullIf.java:57)

Unfortunately I don't have a reproducer (@bpintea probably you know this better than me, if you have ideas about this, I'll be happy to add a query to the tests)

[elasticsearchmachine]

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

[elasticsearchmachine]

Hi @luigidellaquila, I've created a changelog YAML for you.

[bpintea]

LGTM.
I think SELECT NULLIF(PERCENTILE(0+salary+salary, 50), 0) FROM test_emp should do for a test.