[SPARK-17521] Error when I use sparkContext.makeRDD(Seq())

## What changes were proposed in this pull request?

 when i use sc.makeRDD below
```
val data3 = sc.makeRDD(Seq())
println(data3.partitions.length)
```
I got an error:
Exception in thread "main" java.lang.IllegalArgumentException: Positive number of slices required

We can fix this bug just modify the last line ,do a check of seq.size
```
  def makeRDD[T: ClassTag](seq: Seq[(T, Seq[String])]): RDD[T] = withScope {
    assertNotStopped()
    val indexToPrefs = seq.zipWithIndex.map(t => (t._2, t._1._2)).toMap
    new ParallelCollectionRDD[T](this, seq.map(_._1), math.max(seq.size, defaultParallelism), indexToPrefs)
  }
```

## How was this patch tested?

 manual tests

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Author: codlife <1004910847@qq.com>
Author: codlife <wangjianfei15@otcaix.iscas.ac.cn>

Closes #15077 from codlife/master.

1 files changed, 1 insertions, 1 deletions

diff --git a/core/src/main/scala/org/apache/spark/SparkContext.scala b/core/src/main/scala/org/apache/spark/SparkContext.scala
index e32e4aa5b8..35b6334832 100644
--- a/core/src/main/scala/org/apache/spark/SparkContext.scala
+++ b/core/src/main/scala/org/apache/spark/SparkContext.scala
@@ -795,7 +795,7 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
   def makeRDD[T: ClassTag](seq: Seq[(T, Seq[String])]): RDD[T] = withScope {
     assertNotStopped()
     val indexToPrefs = seq.zipWithIndex.map(t => (t._2, t._1._2)).toMap
-    new ParallelCollectionRDD[T](this, seq.map(_._1), seq.size, indexToPrefs)
+    new ParallelCollectionRDD[T](this, seq.map(_._1), math.max(seq.size, 1), indexToPrefs)
   }
 
   /**