## What changes were proposed in this pull request?

Currently, `SQLBuilder` raises `empty.reduceLeft` exceptions on *unoptimized* `EXISTS` queries. We had better prevent this.
```scala
scala> sql("CREATE TABLE t1(a int)")
scala> val df = sql("select * from t1 b where exists (select * from t1 a)")
scala> new org.apache.spark.sql.catalyst.SQLBuilder(df).toSQL
java.lang.UnsupportedOperationException: empty.reduceLeft
```

## How was this patch tested?

Pass the Jenkins tests with a new test suite.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14307 from dongjoon-hyun/SPARK-16672.