Add examples for null as an argument

Author: HyukjinKwon

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

@@ -46,7 +46,7 @@ abstract class RDG extends UnaryExpression with ExpectsInputTypes with Nondeterm
     case Literal(s, IntegerType) => s.asInstanceOf[Int]
     case Literal(s, LongType) => s.asInstanceOf[Long]
     case _ => throw new AnalysisException(
-      s"Input argument to $prettyName must be an integer/long/NULL literal.")
+      s"Input argument to $prettyName must be an integer, long or null literal.")
   }
 
   override def nullable: Boolean = false
@@ -66,6 +66,8 @@ abstract class RDG extends UnaryExpression with ExpectsInputTypes with Nondeterm
        0.9629742951434543
       > SELECT _FUNC_(0);
        0.8446490682263027
+      > SELECT _FUNC_(null);
+       0.8446490682263027
   """)
 // scalastyle:on line.size.limit
 case class Rand(child: Expression) extends RDG {
@@ -99,6 +101,8 @@ object Rand {
        -0.3254147983080288
       > SELECT _FUNC_(0);
        1.1164209726833079
+      > SELECT _FUNC_(null);
+       1.1164209726833079
   """)
 // scalastyle:on line.size.limit
 case class Randn(child: Expression) extends RDG {

sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala

@@ -1635,7 +1635,6 @@ class DataFrameSuite extends QueryTest with SharedSQLContext {
     }
   }
 
-<<<<<<< 5213bd60f4be0795e23362f555dcdcf1a1d060cd
   private def verifyNullabilityInFilterExec(
       df: DataFrame,
       expr: String,