Aggregation of deeply nested window not detected when unparsing

Given a query such as the below:

WITH
  window_cte AS (
    SELECT
      foo,
      SUM(baz) OVER (PARTITION BY foo) AS window_agg
    FROM
      sample_data
  )
SELECT
  SUM(window_agg) AS bazzy
FROM
  window_cte 

When `baz` is optional, Calcite will wrap the inner function within a `CASE` statement. Something like:

CASE
  WHEN (
    COUNT(baz) OVER (
      PARTITION BY
        foo RANGE BETWEEN UNBOUNDED PRECEDING
        AND UNBOUNDED FOLLOWING
    )
  ) > 0 THEN COALESCE(
    SUM(baz) OVER (
      PARTITION BY
        foo
      ORDER BY
        RAND () NULLS LAST RANGE BETWEEN UNBOUNDED PRECEDING
        AND UNBOUNDED FOLLOWING
    ),
    0
  )
  ELSE NULL
END

Nothing intrinsically wrong with that substitution. However because of it the inner aggregation function `SUM(baz) OVER...` has become deeply nested under the `CASE` statement.

Where that becomes an issue is inside `SqlImplemtor#Result#hasNested`. That function is supposed to look for nested aggregates, but it only looks at the top level. In this case it'll stop at the `CASE` statement, erroneously concluding there's no nested aggregate. In the end it'll collapse the cte from the original query into a `SUM(CASE(... SUM(...)))` which will fail running in dialects such as Spark.