[SPARK-8903] Fix bug in cherry-pick of SPARK-8803

This fixes a bug introduced in the cherry-pick of apache#7201 which led to a NullPointerException when cross-tabulating a data set that contains null values.

Author: Josh Rosen <[email protected]>

Closes apache#7295 from JoshRosen/SPARK-8903 and squashes the following commits:

5489948 [Josh Rosen] [SPARK-8903] Fix bug in cherry-pick of SPARK-8803

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

@@ -113,7 +113,7 @@ private[sql] object StatFunctions extends Logging {
       if (element == null) "null" else element.toString
     }
     // get the distinct values of column 2, so that we can make them the column names
-    val distinctCol2: Map[Any, Int] =
+    val distinctCol2: Map[String, Int] =
       counts.map(e => cleanElement(e.get(1))).distinct.zipWithIndex.toMap
     val columnSize = distinctCol2.size
     require(columnSize < 1e4, s"The number of distinct values for $col2, can't " +
@@ -128,7 +128,7 @@ private[sql] object StatFunctions extends Logging {
         countsRow.setLong(columnIndex + 1, row.getLong(2))
       }
       // the value of col1 is the first value, the rest are the counts
-      countsRow.setString(0, cleanElement(col1Item.toString))
+      countsRow.setString(0, cleanElement(col1Item))
       countsRow
     }.toSeq
     // Back ticks can't exist in DataFrame column names, therefore drop them. To be able to accept
@@ -139,7 +139,7 @@ private[sql] object StatFunctions extends Logging {
     // In the map, the column names (._1) are not ordered by the index (._2). This was the bug in
     // SPARK-8681. We need to explicitly sort by the column index and assign the column names.
     val headerNames = distinctCol2.toSeq.sortBy(_._2).map { r =>
-      StructField(cleanColumnName(r._1.toString), LongType)
+      StructField(cleanColumnName(r._1), LongType)
     }
     val schema = StructType(StructField(tableName, StringType) +: headerNames)