SPARK-23927: remove redundant "|boolean ${ev.isNull} = false;"

Author: wajda

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/collectionOperations.scala

@@ -2444,7 +2444,6 @@ case class Sequence(
     } else {
       ev.copy(code =
         code"""
-           |boolean ${ev.isNull} = false;
            |${startGen.code}
            |${stopGen.code}
            |${stepGen.code}