fix: curio: fix incorrect null check for varchar column

[LexLuthr]

Related Issues

Proposed Changes

null check against a varchar column can result in false negatives.

yugabyte=# SELECT * FROM curio.storage_path WHERE available >= 118279372 and NOW()-(50000000000 * INTERVAL '1 second') < last_heartbeat and heartbeat_err is null and ((true and can_seal = TRUE) or (false and can_store = TRUE)) order by (available::numeric * weight) desc;
 storage_id | urls | weight | max_storage | can_seal | can_store | groups | allow_to | allow_types | deny_types | capacity | available | fs_available | reserved | used | last_heartbeat | heartbeat_err | allow_miners | deny_miners 
------------+------+--------+-------------+----------+-----------+--------+----------+-------------+------------+----------+-----------+--------------+----------+------+----------------+---------------+--------------+-------------
(0 rows)

yugabyte=# SELECT * FROM curio.storage_path;
              storage_id              |             urls              | weight | max_storage | can_seal | can_store | groups | allow_to | allow_types | deny_types |   capacity   |  available   | fs_available | reserved | used |       last_heartbeat       | heartbeat_err | allow_miners | deny_miners 
--------------------------------------+-------------------------------+--------+-------------+----------+-----------+--------+----------+-------------+------------+--------------+--------------+--------------+----------+------+----------------------------+---------------+--------------+-------------
 01756dc0-3889-42a8-9b27-c831e9f413f7 | http://127.0.0.1:12300/remote |     10 |           0 | t        | t         |        |          |             |            | 994662584320 | 449571401728 | 449571401728 |        0 |    0 | 2024-04-16 00:17:30.920622 |               |              | 
(1 row)

yugabyte=# SELECT * FROM curio.storage_path WHERE available >= 118279372 and NOW()-(50000000000 * INTERVAL '1 second') < last_heartbeat and heartbeat_err = '' and ((true and can_seal = TRUE) or (false and can_store = TRUE)) order by (available::numeric * weight) desc;
              storage_id              |             urls              | weight | max_storage | can_seal | can_store | groups | allow_to | allow_types | deny_types |   capacity   |  available   | fs_available | reserved | used |       last_heartbeat       | heartbeat_err | allow_miners | deny_miners 
--------------------------------------+-------------------------------+--------+-------------+----------+-----------+--------+----------+-------------+------------+--------------+--------------+--------------+----------+------+----------------------------+---------------+--------------+-------------
 01756dc0-3889-42a8-9b27-c831e9f413f7 | http://127.0.0.1:12300/remote |     10 |           0 | t        | t         |        |          |             |            | 994662584320 | 449591689216 | 449591689216 |        0 |    0 | 2024-04-16 00:20:30.984384 |               |              | 
(1 row)

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