[SPARK-21338][SQL][FOLLOW-UP] Implement isCascadingTruncateTable() method in AggregatedDialect

## What changes were proposed in this pull request?

The implemented `isCascadingTruncateTable` in `AggregatedDialect` is wrong. When no dialect claims cascading, once there is an unknown cascading truncate in the dialects, we should return unknown cascading, instead of false.

## How was this patch tested?

Added test.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #19286 from viirya/SPARK-21338-followup.

sql/core/src/main/scala/org/apache/spark/sql/jdbc/AggregatedDialect.scala

@@ -43,6 +43,12 @@ private class AggregatedDialect(dialects: List[JdbcDialect]) extends JdbcDialect
   }
 
   override def isCascadingTruncateTable(): Option[Boolean] = {
-    dialects.flatMap(_.isCascadingTruncateTable()).reduceOption(_ || _)
+    // If any dialect claims cascading truncate, this dialect is also cascading truncate.
+    // Otherwise, if any dialect has unknown cascading truncate, this dialect is also unknown.
+    dialects.flatMap(_.isCascadingTruncateTable()).reduceOption(_ || _) match {
+      case Some(true) => Some(true)
+      case _ if dialects.exists(_.isCascadingTruncateTable().isEmpty) => None
+      case _ => Some(false)
+    }
   }
 }

sql/core/src/test/scala/org/apache/spark/sql/jdbc/JDBCSuite.scala

@@ -749,6 +749,31 @@ class JDBCSuite extends SparkFunSuite
     assert(agg.isCascadingTruncateTable() === Some(true))
   }
 
+  test("Aggregated dialects: isCascadingTruncateTable") {
+    def genDialect(cascadingTruncateTable: Option[Boolean]): JdbcDialect = new JdbcDialect {
+      override def canHandle(url: String): Boolean = true
+      override def getCatalystType(
+        sqlType: Int,
+        typeName: String,
+        size: Int,
+        md: MetadataBuilder): Option[DataType] = None
+      override def isCascadingTruncateTable(): Option[Boolean] = cascadingTruncateTable
+    }
+
+    def testDialects(cascadings: List[Option[Boolean]], expected: Option[Boolean]): Unit = {
+      val dialects = cascadings.map(genDialect(_))
+      val agg = new AggregatedDialect(dialects)
+      assert(agg.isCascadingTruncateTable() === expected)
+    }
+
+    testDialects(List(Some(true), Some(false), None), Some(true))
+    testDialects(List(Some(true), Some(true), None), Some(true))
+    testDialects(List(Some(false), Some(false), None), None)
+    testDialects(List(Some(true), Some(true)), Some(true))
+    testDialects(List(Some(false), Some(false)), Some(false))
+    testDialects(List(None, None), None)
+  }
+
   test("DB2Dialect type mapping") {
     val db2Dialect = JdbcDialects.get("jdbc:db2://127.0.0.1/db")
     assert(db2Dialect.getJDBCType(StringType).map(_.databaseTypeDefinition).get == "CLOB")