Major Description
There is a bug in the `MergeScalarSubqueries` rule for queries with subquery expressions nested inside each other, wherein the rule attempts to merge the nested subquery with its enclosing parent subquery. The result is not a valid plan and raises an exception in the optimizer. Here is a minimal reproducing case:
```
sql("create table test(col int) using csv")
checkAnswer(sql("select(select sum((select sum(col) from test)) from test)"), Row(null))
```
