[CARBONDATA-4214] inserting NULL value when timestamp value received from FROM_UNIXTIME(0)

Why is this PR needed?
Filling null in case of timestamp value is received from FROM_UNIXTIME(0) as spark original
insert rdd value[internalRow] received in this case zero. if the original column
value[internalRow] is zero then in insert flow adding NULL and giving NULL to spark.
When query happens on the same column received NULL value instead of timestamp value.
Problem code: if (internalRow.getLong(index) == 0) { internalRow.setNullAt(index) }

What changes were proposed in this PR?
Removed the null filling check for zero value case and if internalRow value is non
null/empty then only set the internalRow timestamp value.

Does this PR introduce any user interface change?
No

Is any new testcase added?
Yes

This closes apache#4154

Author: maheshrajus

integration/spark/src/main/scala/org/apache/spark/sql/execution/command/management/CommonLoadUtils.scala

@@ -781,9 +781,8 @@ object CommonLoadUtils {
         internalRowOriginal
       }
       for (index <- timeStampIndex) {
-        if (internalRow.getLong(index) == 0) {
-          internalRow.setNullAt(index)
-        } else {
+        // timestmap value can be set other than null/empty case
+        if (!internalRow.isNullAt(index)) {
           internalRow.setLong(
             index,
             internalRow.getLong(index) / TimeStampGranularityTypeValue.MILLIS_SECONDS.getValue)

integration/spark/src/test/scala/org/apache/carbondata/spark/testsuite/allqueries/AllDataTypesTestCase.scala

@@ -17,6 +17,8 @@
 
 package org.apache.carbondata.spark.testsuite.allqueries
 
+import java.sql.Timestamp
+
 import org.apache.spark.sql.{Row, SaveMode}
 import org.apache.spark.sql.test.util.QueryTest
 import org.scalatest.BeforeAndAfterAll
@@ -73,6 +75,33 @@ class AllDataTypesTestCase extends QueryTest with BeforeAndAfterAll {
     }
   }
 
+  test("insert data with from_unixtime(0) and query") {
+    sql("drop table if exists time_carbon1")
+    sql("drop table if exists time_parquet")
+    sql("create table if not exists time_carbon1(time1 timestamp) stored as carbondata")
+    sql("create table if not exists time_parquet(time1 timestamp) stored as parquet")
+    sql("insert into time_carbon1 select from_unixtime(0)")
+    sql("insert into time_parquet select from_unixtime(0)")
+    sql("insert into time_carbon1 select from_unixtime(123456)")
+    sql("insert into time_parquet select from_unixtime(123456)")
+    checkAnswer(sql("select * from time_carbon1"),
+      sql("select * from time_parquet"))
+    sql("drop table if exists time_carbon1")
+    sql("drop table if exists time_parquet")
+  }
+
+  test("insert data with empty/null and query") {
+    sql("drop table if exists time_carbon2")
+    sql("create table if not exists time_carbon2(time1 timestamp) stored as carbondata")
+    sql("insert into time_carbon2 select null")
+    checkAnswer(sql("select count(*) from time_carbon2"), Seq(Row(1)))
+    checkAnswer(sql("select * from time_carbon2"), Seq(Row(null)))
+    sql("insert into time_carbon2 select ''")
+    checkAnswer(sql("select count(*) from time_carbon2"), Seq(Row(2)))
+    checkAnswer(sql("select * from time_carbon2"), Seq(Row(null), Row(null)))
+    sql("drop table if exists time_carbon2")
+  }
+
   // Test-24
   test("select channelsId, sum(channelsId+ 10)  Total from Carbon_automation_test group by  channelsId order by Total") {
     checkAnswer(