[#23322] YSQL: pg_partman: fix logic of checking existing table

Summary:
Diff D36428 added an incorrect assertion in create_partition_time to check if a child table is already created .

```
    IF v_exists IS NULL THEN
        RAISE DEBUG 'create_partition_time v_sql: %', v_sql;
        EXECUTE v_sql;
    END IF;
```

`v_exists` is of type `smallint` and should be check if it is greater than zero or not. This diff fixes this bug.
Jira: DB-12247

Test Plan:
jenkins: compile only

Manual Test:

```
yugabyte=# CREATE SCHEMA IF NOT EXISTS partman;
CREATE SCHEMA
yugabyte=# CREATE EXTENSION pg_partman WITH SCHEMA partman;
CREATE EXTENSION
yugabyte=# CREATE SCHEMA partman_test;
CREATE SCHEMA
yugabyte=# CREATE TABLE partman_test.time_taptest_table (
yugabyte(#     col1 int,
yugabyte(#     col2 text,
yugabyte(#     col3 int NOT NULL DEFAULT extract('epoch' from CURRENT_TIMESTAMP)::int)
yugabyte-# PARTITION BY RANGE (col3);
CREATE TABLE
yugabyte=# \d partman_test.time_taptest_table
                             Table "partman_test.time_taptest_table"
 Column |  Type   | Collation | Nullable |                        Default
--------+---------+-----------+----------+--------------------------------------------------------
 col1   | integer |           |          |
 col2   | text    |           |          |
 col3   | integer |           | not null | (date_part('epoch'::text, CURRENT_TIMESTAMP))::integer
Partition key: RANGE (col3)
Number of partitions: 0

yugabyte=# SELECT partman.create_parent('partman_test.time_taptest_table', 'col3', 'native', 'weekly', '{"col1"}', p_epoch := 'seconds'
yugabyte(#     , p_premake := 2, p_start_partition := to_char(CURRENT_TIMESTAMP-'8 weeks'::interval, 'YYYY-MM-DD HH24:MI:SS'));
 create_parent
---------------
 t
(1 row)

yugabyte=# \d+ partman_test.time_taptest_table
                                                 Table "partman_test.time_taptest_table"
 Column |  Type   | Collation | Nullable |                        Default                         | Storage  | Stats target | Description
--------+---------+-----------+----------+--------------------------------------------------------+----------+--------------+-------------
 col1   | integer |           |          |                                                        | plain    |              |
 col2   | text    |           |          |                                                        | extended |              |
 col3   | integer |           | not null | (date_part('epoch'::text, CURRENT_TIMESTAMP))::integer | plain    |              |
Partition key: RANGE (col3)
Partitions: partman_test.time_taptest_table_p2024w23 FOR VALUES FROM (1717353000) TO (1717957800),
            partman_test.time_taptest_table_p2024w24 FOR VALUES FROM (1717957800) TO (1718562600),
            partman_test.time_taptest_table_p2024w25 FOR VALUES FROM (1718562600) TO (1719167400),
            partman_test.time_taptest_table_p2024w26 FOR VALUES FROM (1719167400) TO (1719772200),
            partman_test.time_taptest_table_p2024w27 FOR VALUES FROM (1719772200) TO (1720377000),
            partman_test.time_taptest_table_p2024w28 FOR VALUES FROM (1720377000) TO (1720981800),
            partman_test.time_taptest_table_p2024w29 FOR VALUES FROM (1720981800) TO (1721586600),
            partman_test.time_taptest_table_p2024w30 FOR VALUES FROM (1721586600) TO (1722191400),
            partman_test.time_taptest_table_p2024w31 FOR VALUES FROM (1722191400) TO (1722796200),
            partman_test.time_taptest_table_p2024w32 FOR VALUES FROM (1722796200) TO (1723401000),
            partman_test.time_taptest_table_p2024w33 FOR VALUES FROM (1723401000) TO (1724005800),
            partman_test.time_taptest_table_default DEFAULT
```

Reviewers: jason, hsunder

Reviewed By: hsunder

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D36916

src/postgres/third-party-extensions/pg_partman/sql/functions/create_partition_time.sql

@@ -259,7 +259,7 @@ FOREACH v_time IN ARRAY p_partition_times LOOP
         END IF;
     END IF;
 
-    IF v_exists IS NULL THEN
+    IF v_exists = 0 THEN
         RAISE DEBUG 'create_partition_time v_sql: %', v_sql;
         EXECUTE v_sql;
     END IF;