[SPARK-15709][SQL] Prevent freqItems from raising `UnsupportedOpera…

…tionException: empty.min`

## What changes were proposed in this pull request?

Currently, `freqItems` raises `UnsupportedOperationException` on `empty.min` usually when its `support` argument is high.
```scala
scala> spark.createDataset(Seq(1, 2, 2, 3, 3, 3)).stat.freqItems(Seq("value"), 2)
16/06/01 11:11:38 ERROR Executor: Exception in task 5.0 in stage 0.0 (TID 5)
java.lang.UnsupportedOperationException: empty.min
...
```

Also, the parameter checking message is wrong.
```
require(support >= 1e-4, s"support ($support) must be greater than 1e-4.")
```

This PR changes the logic to handle the `empty` case and also improves parameter checking.

## How was this patch tested?

Pass the Jenkins tests (with a new testcase).

Author: Dongjoon Hyun <[email protected]>

Closes apache#13449 from dongjoon-hyun/SPARK-15709.

sql/core/src/main/scala/org/apache/spark/sql/execution/stat/FrequentItems.scala

@@ -40,7 +40,7 @@ private[sql] object FrequentItems extends Logging {
         if (baseMap.size < size) {
           baseMap += key -> count
         } else {
-          val minCount = baseMap.values.min
+          val minCount = if (baseMap.values.isEmpty) 0 else baseMap.values.min
           val remainder = count - minCount
           if (remainder >= 0) {
             baseMap += key -> count // something will get kicked out, so we can add this
@@ -83,7 +83,7 @@ private[sql] object FrequentItems extends Logging {
       df: DataFrame,
       cols: Seq[String],
       support: Double): DataFrame = {
-    require(support >= 1e-4, s"support ($support) must be greater than 1e-4.")
+    require(support >= 1e-4 && support <= 1.0, s"Support must be in [1e-4, 1], but got $support.")
     val numCols = cols.length
     // number of max items to keep counts for
     val sizeOfMap = (1 / support).toInt

sql/core/src/test/scala/org/apache/spark/sql/DataFrameStatSuite.scala

@@ -235,6 +235,17 @@ class DataFrameStatSuite extends QueryTest with SharedSQLContext {
     assert(items.length === 1)
   }
 
+  test("SPARK-15709: Prevent `UnsupportedOperationException: empty.min` in `freqItems`") {
+    val ds = spark.createDataset(Seq(1, 2, 2, 3, 3, 3))
+
+    intercept[IllegalArgumentException] {
+      ds.stat.freqItems(Seq("value"), 0)
+    }
+    intercept[IllegalArgumentException] {
+      ds.stat.freqItems(Seq("value"), 2)
+    }
+  }
+
   test("sampleBy") {
     val df = spark.range(0, 100).select((col("id") % 3).as("key"))
     val sampled = df.stat.sampleBy("key", Map(0 -> 0.1, 1 -> 0.2), 0L)