[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 <aihe@usc.edu>
Author: aihe@usc.edu <aihe@usc.edu>

Closes #5810 from AiHe/decisiontree-issue and squashes the following commits:

3b1d08a [aihe@usc.edu] [MLLIB][tree] merge the assertion into the evaluation of numFeatures
cf2e567 [Alain] [MLLIB][tree] Use a rdd api to verify size of input rdd > 0 when building meta data
b448f47 [Alain] [MLLIB][tree] Verify size of input rdd > 0 when building meta data

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

@@ -107,7 +107,10 @@ private[tree] object DecisionTreeMetadata extends Logging {
       numTrees: Int,
       featureSubsetStrategy: String): DecisionTreeMetadata = {
 
-    val numFeatures = input.take(1)(0).features.size
+    val numFeatures = input.map(_.features.size).take(1).headOption.getOrElse {
+      throw new IllegalArgumentException(s"DecisionTree requires size of input RDD > 0, " +
+        s"but was given by empty one.")
+    }
     val numExamples = input.count()
     val numClasses = strategy.algo match {
       case Classification => strategy.numClasses