[SPARK-23001][SQL] Fix NullPointerException when DESC a database with NULL description

## What changes were proposed in this pull request?
When users' DB description is NULL, users might hit `NullPointerException`. This PR is to fix the issue.

## How was this patch tested?
Added test cases

Author: gatorsmile <gatorsmile@gmail.com>

Closes #20215 from gatorsmile/SPARK-23001.

sql/hive/src/main/scala/org/apache/spark/sql/hive/client/HiveClientImpl.scala

@@ -330,7 +330,7 @@ private[hive] class HiveClientImpl(
     Option(client.getDatabase(dbName)).map { d =>
       CatalogDatabase(
         name = d.getName,
-        description = d.getDescription,
+        description = Option(d.getDescription).getOrElse(""),
         locationUri = CatalogUtils.stringToURI(d.getLocationUri),
         properties = Option(d.getParameters).map(_.asScala.toMap).orNull)
     }.getOrElse(throw new NoSuchDatabaseException(dbName))

sql/hive/src/test/scala/org/apache/spark/sql/hive/HiveExternalCatalogSuite.scala

@@ -107,4 +107,10 @@ class HiveExternalCatalogSuite extends ExternalCatalogSuite {
       .filter(_.contains("Num Buckets")).head
     assert(bucketString.contains("10"))
   }
+
+  test("SPARK-23001: NullPointerException when running desc database") {
+    val catalog = newBasicCatalog()
+    catalog.createDatabase(newDb("dbWithNullDesc").copy(description = null), ignoreIfExists = false)
+    assert(catalog.getDatabase("dbWithNullDesc").description == "")
+  }
 }

sql/hive/src/test/scala/org/apache/spark/sql/hive/client/VersionsSuite.scala

@@ -163,6 +163,15 @@ class VersionsSuite extends SparkFunSuite with Logging {
       client.createDatabase(tempDB, ignoreIfExists = true)
     }
 
+    test(s"$version: createDatabase with null description") {
+      withTempDir { tmpDir =>
+        val dbWithNullDesc =
+          CatalogDatabase("dbWithNullDesc", description = null, tmpDir.toURI, Map())
+        client.createDatabase(dbWithNullDesc, ignoreIfExists = true)
+        assert(client.getDatabase("dbWithNullDesc").description == "")
+      }
+    }
+
     test(s"$version: setCurrentDatabase") {
       client.setCurrentDatabase("default")
     }