Use null schema response for SYS TABLES command. (#46386)

(cherry picked from commit a6152f4)

Author: astefan

x-pack/plugin/sql/qa/src/main/resources/setup_mock_metadata_get_tables.sql

@@ -9,7 +9,7 @@ CREATE TABLE mock (
   SELF_REFERENCING_COL_NAME VARCHAR,
   REF_GENERATION VARCHAR
 ) AS
-SELECT '', 'test1', 'BASE TABLE', '', null, null, null, null, null FROM DUAL
+SELECT null, 'test1', 'BASE TABLE', '', null, null, null, null, null FROM DUAL
 UNION ALL
-SELECT '', 'test2', 'BASE TABLE', '', null, null, null, null, null FROM DUAL
+SELECT null, 'test2', 'BASE TABLE', '', null, null, null, null, null FROM DUAL
 ;

x-pack/plugin/sql/qa/src/main/resources/setup_mock_metadata_get_types_of_table.sql

@@ -9,7 +9,7 @@ CREATE TABLE mock (
   SELF_REFERENCING_COL_NAME VARCHAR,
   REF_GENERATION VARCHAR
 ) AS
-SELECT '', 'test1', 'BASE TABLE', '', null, null, null, null, null FROM DUAL
+SELECT null, 'test1', 'BASE TABLE', '', null, null, null, null, null FROM DUAL
 UNION ALL
-SELECT '', 'test2', 'BASE TABLE', '', null, null, null, null, null FROM DUAL
+SELECT null, 'test2', 'BASE TABLE', '', null, null, null, null, null FROM DUAL
 ;

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/plan/logical/command/sys/SysTables.java

@@ -146,7 +146,7 @@ public final void execute(SqlSession session, ActionListener<Page> listener) {
                  .sorted(Comparator.<IndexInfo, String> comparing(i -> legacyName(i.type()))
                            .thenComparing(Comparator.comparing(i -> i.name())))
                  .map(t -> asList(cluster,
-                         EMPTY,
+                         null,
                          t.name(),
                          legacyName(t.type()),
                          EMPTY,