add null check in DateExpressionSuite

Author: yjshen

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/DateExpressionsSuite.scala

@@ -106,6 +106,7 @@ class DateExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
         }
       }
     }
+    checkEvaluation(DayOfYear(Literal.create(null, DateType)), null)
   }
 
   test("Year") {
@@ -274,6 +275,7 @@ class DateExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     checkEvaluation(LastDay(Literal(Date.valueOf("2015-12-05"))), Date.valueOf("2015-12-31"))
     checkEvaluation(LastDay(Literal(Date.valueOf("2016-01-06"))), Date.valueOf("2016-01-31"))
     checkEvaluation(LastDay(Literal(Date.valueOf("2016-02-07"))), Date.valueOf("2016-02-29"))
+    checkEvaluation(LastDay(Literal.create(null, DateType)), null)
   }
 
   test("next_day") {