Fix NULL predicate error in table model without TAG columns

[Young-Leo]

This fixes a table-model issue where NULL-related predicates fail only
when the table has no TAG columns, while the same predicates work when
TAG columns exist.

Repro (table model)

Table without TAG (used to fail)

CREATE TABLE test_table_no_tag(
  id INT32 FIELD,
  name STRING FIELD,
  value DOUBLE FIELD
);

SELECT * FROM test_table_no_tag WHERE name = null;

Table with TAG (used to work)

CREATE TABLE test_table_with_tag(
  id STRING TAG,
  name STRING FIELD,
  value DOUBLE FIELD
);

SELECT * FROM test_table_with_tag WHERE name = null;

Fix

Handle casted NULL predicates across predicate visitors so the no-TAG
path no longer fails.

[JackieTien97]

not right, what if I write CAST(NULL AS BOOLEAM) IS NULL ?