[SPARK-28930][SQL] Last Access Time value shall display 'UNKNOWN' as currently system cannot evaluate the last access time, and 'null' values will be shown in its capital form 'NULL' for SQL client to make the

display format similar to spark-shell.

What changes were proposed in this pull request?
If there is no comment for spark scala shell shows "null" in small letters but all other places Hive beeline/Spark beeline/Spark SQL it is showing in CAPITAL "NULL". In this patch
shown in its capital form 'NULL' for SQL client to make the display format similar to Hive beeline/Spark beeline/Spark SQL. Also corrected the Last Access time, the value shall display 'UNKNOWN'
as currently system wont support the last access time evaluation.
Issue 2 mentioned in JIRA Spark SQL "desc formatted tablename" is not showing the header # col_name,data_type,comment , seems to be the header has been removed knowingly as part of SPARK-20954.

Does this PR introduce any user-facing change?
No

How was this patch tested?
Locally and corrected a ut.

Author: sujith71955

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/trees/TreeNodeSuite.scala

@@ -513,7 +513,7 @@ class TreeNodeSuite extends SparkFunSuite with SQLHelper {
         "partitionColumnNames" -> List.empty[String],
         "owner" -> "",
         "createTime" -> 0,
-        "lastAccessTime" -> -1,
+        "lastAccessTime" -> 0,
         "createVersion" -> "2.x",
         "tracksPartitionsInCatalog" -> false,
         "properties" -> JNull,

sql/core/src/main/scala/org/apache/spark/sql/execution/command/tables.scala

@@ -510,8 +510,7 @@ abstract class DescribeCommandBase extends RunnableCommand {
       append(buffer, s"# ${output.head.name}", output(1).name, output(2).name)
     }
     schema.foreach { column =>
-      append(buffer, column.name, column.dataType.simpleString,
-        column.getComment().getOrElse("NULL"))
+      append(buffer, column.name, column.dataType.simpleString, column.getComment().orNull)
     }
   }
 

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveDDLSuite.scala

@@ -1042,11 +1042,11 @@ class HiveDDLSuite
 
       assert(sql("DESC tbl").collect().containsSlice(
         Seq(
-          Row("a", "int", "NULL"),
-          Row("b", "int", "NULL"),
+          Row("a", "int", null),
+          Row("b", "int", null),
           Row("# Partition Information", "", ""),
           Row("# col_name", "data_type", "comment"),
-          Row("b", "int", "NULL")
+          Row("b", "int", null)
         )
       ))
     }
@@ -1617,7 +1617,7 @@ class HiveDDLSuite
 
           val desc = sql("DESC FORMATTED t1").collect().toSeq
 
-          assert(desc.contains(Row("id", "bigint", "NULL")))
+          assert(desc.contains(Row("id", "bigint", null)))
         }
       }
     }