[SPARK-16302][SQL] Set the right number of partitions for reading data from a local collection.

follow #13137 This pr sets the right number of partitions when reading data from a local collection.
Query 'val df = Seq((1, 2)).toDF("key", "value").count' always use defaultParallelism tasks. So it causes run many empty or small tasks.

Manually tested and checked.

Author: Lianhui Wang <lianhuiwang09@gmail.com>

Closes #13979 from lianhuiwang/localTable-Parallel.

sql/core/src/main/scala/org/apache/spark/sql/execution/LocalTableScanExec.scala

@@ -42,7 +42,10 @@ case class LocalTableScanExec(
     }
   }
 
-  private lazy val rdd = sqlContext.sparkContext.parallelize(unsafeRows)
+  private lazy val numParallelism: Int = math.min(math.max(unsafeRows.length, 1),
+    sqlContext.sparkContext.defaultParallelism)
+
+  private lazy val rdd = sqlContext.sparkContext.parallelize(unsafeRows, numParallelism)
 
   protected override def doExecute(): RDD[InternalRow] = {
     val numOutputRows = longMetric("numOutputRows")