bug(frontend): wrong cast for `NULL` in places we require `boolean`

[kwannoel]

Describe the bug

dev=> SELECT 1 having null;
ERROR:  QueryError: internal error: argument of HAVING must be boolean, not type Varchar
dev=>

To Reproduce

SELECT 1 having null;

Expected behavior

postgres=# select 1 having null;
 ?column?
----------

Additional context

No response

[xiangjinwu]

Basically, at places where we check expr.return_type() == Bool (incl. where and having), we should check expr.is_unknown() and then expr.cast_implicit(Bool). Note the function is is_unknown rather than is_null, as the former also handles SELECT 1 where 'y';