[SPARK-8140] [MLLIB] Remove construct to get weights in StreamingLinearAlgorithm

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #6720 from MechCoder/empty_model_check and squashes the following commits:

3a07de5 [MechCoder] Remove construct to get weights in StreamingLinearAlgorithm

mllib/src/main/scala/org/apache/spark/mllib/regression/StreamingLinearAlgorithm.scala

@@ -83,12 +83,7 @@ abstract class StreamingLinearAlgorithm[
       throw new IllegalArgumentException("Model must be initialized before starting training.")
     }
     data.foreachRDD { (rdd, time) =>
-      val initialWeights =
-        model match {
-          case Some(m) =>
-            m.weights
-        }
-      model = Some(algorithm.run(rdd, initialWeights))
+      model = Some(algorithm.run(rdd, model.get.weights))
       logInfo("Model updated at time %s".format(time.toString))
       val display = model.get.weights.size match {
         case x if x > 100 => model.get.weights.toArray.take(100).mkString("[", ",", "...")