[MLLIB][tree] Verify size of input rdd > 0 when building meta data

Require non empty input rdd such that we can take the first
labeledpoint and get the feature size

Author: Alain

mllib/src/main/scala/org/apache/spark/mllib/tree/impl/DecisionTreeMetadata.scala

@@ -107,8 +107,11 @@ private[tree] object DecisionTreeMetadata extends Logging {
       numTrees: Int,
       featureSubsetStrategy: String): DecisionTreeMetadata = {
 
-    val numFeatures = input.take(1)(0).features.size
     val numExamples = input.count()
+    require(numExamples > 0, s"DecisionTree requires size of input RDD > 0, " +
+    s"but was given by an empty one.")
+
+    val numFeatures = input.take(1)(0).features.size
     val numClasses = strategy.algo match {
       case Classification => strategy.numClasses
       case Regression => 0