ImplicitCastException and stuck when comparing STRING with NULL

[YuanchengJiang]

Describe the bug

QuestDB stuck when comparing STRING type variables with NULL.
When the comparison is in the predicate, it gives the Exception:

AsyncFilteredRecordCursor unexpected error [ex=
io.questdb.cairo.ImplicitCastException: inconvertible value: `INTC` [STRING -> CHAR]
]

To reproduce

create table test(time TIMESTAMP NOT NULL, symbol STRING NOT NULL);
insert into test values ('2023-04-14 19:10:48.002','INTC');
select 1 from test where test.symbol > NULL LIMIT 1; // Exception
select test.symbol > NULL from test LIMIT 1; // Stuck

Expected Behavior

No response

Environment

- **QuestDB version**: 7.1.3
- **OS**: ubuntu 20.04

Additional context

No response